The three Iranian Fast Attack Craft sped towards HMS Dragon from the port side. Nimble, quick and armed with machine guns, the small vessels darted towards the Royal Navy Type 45 Destroyer, threatening to weave between her and the three other British vessels she was escorting through the Strait of Hormuz in the Gulf. Although dwarfed by the British ships, if the Iranians had malign intent they could still cause a lot of damage.

The Revolutionary Guards on the speed boats filmed the British reaction as they raced over the calm sea, looking for any signs of weakness or sloppy drills. Finding neither.

HMS Dragon’s Captain ordered five deafening blasts of the ship’s horn – the international warning signal – as his crew donned helmets and body armour and manned the ship’s heavy machine guns; an escalatory but proportionate response. The Officer of the Watch called the Sapan Navy, the name for the naval element of the Iranian Revolutionary Guards, over the radio.

“Sapan navy, Sapan navy. This is Coalition warship. Despite assuring me you would remain 4000 yards clear of my position you have impeded my passage. Request you remain 1000 yards clear of my port side. Do you understand? Over.”

“For your information we are here to safeguard our national sovereignty and we are doing routine operations. Over,” came the reply.

HMS Dragon reduced speed but held her course and the Iranian craft moved aside. The tension slowly eased as they headed back to their island base, just off the coast of Iran.

“You have to provide a robust response,” Commander Mike Carter-Quinn, the ship’s Captain says. “They will push you. If required we could step up the warnings to include flares and ultimately the use of warning shots to make it very clear that we intend to protect ourselves and continue our right to proceed with our task”.

It was a routine encounter between Britain and Iran on the high seas, repeated on almost every passage through the Straits. But there is no room for complacency. “The day you don’t take it seriously is the day you become unstuck,” says the Captain.

The British crew regard such behaviour as normal. Many use identical words to describe the Iranian conduct: safe and professional, a diplomatic if slightly schooled response to my enquiries.

The Royal Navy is charged with projecting British power around the globe every day of the year. That means it occasionally has to sail a delicate diplomatic course through choppy waters. “The contest between Iran and Saudi Arabia provides the backdrop to everything out here,” says Lieutenant Commander Richard Attwater, HMS Dragon’s second-in-command. “It’s a very volatile region. And we’re sailing 8,000 tonnes of Britain right through the middle of it.”

Britain has four Royal Navy Minehunters and a Royal Fleet Auxiliary (RFA) logistics vessel permanently based in Bahrain. The principal mission of the Minehunters is to ensure safe passage of Britain’s nuclear armed submarines from their base in Scotland out to the North Atlantic. But in recent years the fleet of 13 vessels has also been used to keep open the shipping lanes in the Gulf – and especially the narrow Straits of Hormuz, through which one fifth of the world’s oil is carried.

The Gulf has been a flashpoint for decades. The politics of the region are intricate, frustrating and glacial; outsiders must tread carefully. Britain has friends here, more since America’s pivot to Asia has left many Gulf states questioning the US commitment to the region. But old enmities persist.

“Iran assumes the UK is behind everything that goes wrong for them,” says Professor Michael Clarke of the Royal United Services Institute. “They see the United States as the Great Satan and the UK as the Little Satan. They assume we have immense influence with the US and take us far more seriously that we take ourselves.”

It means Britain can be a conduit to mainstream politics in the Gulf, Mr Clarke believes, but it also means Iran pays extra attention to British ships in the region. “They use the Gulf as a tap to turn on the tension. Anything that happens is not by accident. They use the Gulf as a valve to indicate their degree of cooperation [with the international community] and want to create a sense of danger.”

The big danger right now is the potential collapse of the Iran nuclear deal after Donald Trump’s exit has left it hanging by a thread. There is a real chance of conflict if it fails. Britain is seen by Iran both as a means of influencing America and as the gatekeeper to the other European signatories to the deal.

Iran may decide the wisest course right now is to let events run as they are; an overly aggressive posture towards Britain could be counter-productive. Or it may sense an opportunity to pull at the threads that tie Western powers to the Gulf.

Two Minehunters and an RFA logistics vessel push on through these turbulent geo-political seas, escorted by HMS Dragon, with her twin gas-turbine generators producing enough energy to power a city the size of Leicester, and her air defence radar looking out hundreds of miles for threats from the sky. “We don’t yet understand the full capability of the radar,” says the ships Operations Officer. “It’s like a spaniel straining on a leash.”

Maintaining a naval commitment in the face of these dynamics is costly and dangerous. Why does Britain do it?

“We can’t afford not to,” says Commander Ashley Spencer, head of the Mine Counter Measures fleet. Britain’s naval contribution to the 33-nation Maritime Coalition Force based in Bahrain helps to ensure safe passage on the high seas from the Suez Canal to the Indian Ocean. This global response is a clear indication of how the world values the continued free movement of trade, 90 per cent of which goes across the oceans.

Britain receives a delivery of Liquified Natural Gas every three days from Qatar to the terminal in Wales, to say nothing of the country’s dependence on oil and containerised goods from elsewhere in the world. If that supply chain were disrupted the effects would be felt very rapidly. “Humans only act out of fear, honour or interest,” Commander Spencer says. “And influencing this region is definitely in Britain’s interest.”

The main contribution the Royal Navy makes is through the Minehunting force. HMS Ledbury and HMS Shoreham are 60m and 53m long respectively. Neither has a crew over 50 souls. But they deliver world-leading mine survey, detection and destruction capabilities. Their fixed (Ledbury) and variable-depth (Shoreham) sonars can see objects as small as tin cans. If a mine has been laid, they will find it.

The hulls of both Minehunters are made of glass reinforced plastic instead of steel like bigger ships. It makes for quieter running and enables the crews to find magnetic and acoustic mines without setting them off. Once given a task, each crew must first test the water for temperature and salinity in order to better employ the ship’s sonars.

When a suspicious object has been identified the crew release an unmanned drone to get a closer look. Seafox, a fibre optic wire-guided mine-hunting submersible controlled from the mother ship, weighs 42kgs and measures just over 1.3m long. Operating down to 270m and up to a kilometre from the ship, it uses its own sonar to identify the exact location of its target, then switches on a light and TV camera so the controller back on the ship can assess whether the target is a mine, a rock or a piece of debris. It’s not unusual to find shopping trolleys on the sea bed, even in the Gulf. Seafox can then destroy the mine by detonating an explosive charge, shaped so as to blast through any armour plating.

Divers can go down to place similar charges if the environment precludes the use of Seafox. A lot of the divers’ training is done in zero visibility back in the UK, so they are used to murky and sandy conditions. But some modern mines will react to the metal parts in dive sets, or the sound of expanding air bubbles exhaled by divers. For these reasons the clearance divers use special diving kits called CDLSE, or clearance divers life support equipment, a mixed gas rebreather set which combines oxygen and helium.

The air breathed out is scrubbed of CO2 then has oxygen added before it is reused by the diver. The sets are quiet and have very low magnetic signatures. But diving is an inherently dangerous task so each Minehunter also has a transportable manned compression chamber for diving casualties. The injured diver is put in the chamber and by the injection of compressed air is effectively taken back down to depth. This causes the inert gases within the blood to compress, at which point they can be flushed out of the diver’s body with up to 100 per cent pure oxygen.

“Diving is a dangerous operation, but for us it’s just a mode of transport to get us to the job that we need to conduct,” says Lieutenant James Dutt, second-in-command of HMS Shoreham.

The little flotilla of ships safely broke out of the Gulf and headed south-west across the Arabian Sea, towards the Omani deep-water port of Duqm to take part in the maritime phase of Exercise Saif Sareea III.

From the bridge of the RFA ship Cardigan Bay we watch as dolphins play through the waves just behind the vessel on the left-hand side; the port quarter in naval terminology. Numbering about 2,000 sailors, the RFA is a civilian fleet of mariners in direct support of the Royal Navy, a role it has played since the Napoleonic wars. The barrel of brandy preserving Admiral Nelson’s body, in transport back to the UK after his death at the battle of Trafalgar, was probably supplied to the Navy by an RFA ship, Captain Jed MacAnley tells me.

RFA ships carry weapons for self-defence, but are dual-roled to provide an amphibious assault capability for the Royal Marines. As such, some adversaries may consider them as combatants; fair game when the shooting starts. The government clarified their legal status a few years ago and RFA personnel are now deemed ‘sponsored reserves’; civilians still, but a uniformed, fighting arm of the Royal Navy, subject to military discipline and thereby protected by the Geneva Conventions when working. The change to their terms of service were not all for the better though. “I can no longer shoot malingerers,” Captain MacAnley says with a grin.

Exercise Saif Sareea III sees 65,000 Omani troops join 5,000 British military personnel, tanks, planes and ships in a huge effort to practice desert warfare. It is also a demonstration of commitment, by both countries. Duqm port, the off-loading point, has 2km-long jetties abutting very deep water. Were Britain’s new aircraft carrier, HMS Queen Elizabeth, to come this way in a few years time, Duqm is one of a very small number of places she could berth alongside.

The point is not lost on the Omanis. Outside the port, heading towards the month-old international airport on the traffic-free three-lane carriageway of Airport Road, large wooden signs announce ‘education zone’ or ‘health zone’ to the empty expanse of rocky desert. This place will never draw the tourists like Dubai – the regular south-west monsoon will see to that – but it seeks to draw huge investment and become an economic hub. China has already signed up.

For her part, Britain is seeking to deepen the already strong ties with Oman, and a military staging post such as Duqm, next to a huge and civilian-free training area, is very attractive. The opportunity to exercise on such scale has left Commodore James Parkin, the Amphibious Task Group Commander, in a state of child-like excitement, he says.

Speaking on board HMS Albion, one of Britain’s two dedicated amphibious assault ships, Commodore Parkin extols the virtues of amphibious warfare. “I am swift and silent and can deliver multiple effects from Royal Marines ashore,” he says. Tim Nield, Captain of HMS Albion, agrees. “We can sit 12.1 nautical miles offshore [just outside a country’s territorial limit], completely legally and for a long time,” he says. “A day later, I can be 300 miles away.”

The ability to loiter with the capability to interdict shipping, raid ashore or provide humanitarian relief to a disaster zone focusses political and military minds, friends and foe alike.

“A ship of this size gives us the flexibility to be able to configure ourselves for whatever mission we need to do,” Royal Marine Major James Smith tells me, from the floor of the well dock. Once flooded, the dock can hold four Landing Craft Utility vessels, each with up to 100 Royal Marines and four armoured vehicles on board, ready to go into action.

This is the Royal Navy in 2018: on operations every day of the year, balancing political ambition with military reality and demanding a mental agility from its people to switch seamlessly from delivering hard to soft power depending on the task. It is not without its problems: tours at sea are generally longer than in the past to make best use of the limited number of personnel and the connection to society, so vital for recruitment and national goodwill, has never been more slender.

But there is also cause for much optimism. The Minehunters, Britain’s tupperware warriors, in the words of their commander, are the envy of the world. Maintaining trade routes in the face of regular provocation, they offer an example of how shared risk and enterprise can be mutually beneficial, in an arena far from home. Equally Duqm, access to which was gained by decades of close friendship and commitment to Oman, may well be a future staging post for Britain to exert influence in the uncertain post-Brexit world. But some things are timeless. “Why did I join the Navy?,” one sailor answers me. “I’ve seen the sun set in a thousand places. How many people can say that?”

This article was edited and printed in the Telegraph and online on October 23rd