Five years ago, Prime Minister David Cameron promised the “greenest government ever”. Anyone living among the coalition’s newly encouraged sprawl of houses into Britain’s rural villages has been wondering wryly what he meant. There is next to no work for the new arrivals in green country, so they take to their polluting cars to find a job in town. Those images of Cameron on a London bicycle seem a hollow irrelevancy. Now, against expectations, the promise has been fulfilled, not on land but at sea.

In March, the 2015 Budget statement announced a new Marine Protected Area in about 320,000 sq miles of British territory. Backbench Tory MPs Zac Goldsmith and Nick Hurd have been among the longstanding champions of the measure. When the Budget’s small print was released, they hailed it as the greenest proposal ever by any UK government. They are right — at the eleventh-hour, blue-green Cameron has come good. He has now capped it in the Tories’ election manifesto. It proposes another 13 “blue belts”, which will put Britain at the top of the world’s marine protectors.

The Budget’s protected zone will surround an outlying bit of Britannia, the Pitcairn Islands in the South Pacific. Like the Falklands, the four Pitcairn Islands are British territory, which most of us never knew we still controlled. They came into our orbit because of the Mutiny on the Bounty, whose 226th anniversary fell only last weekend. When its bloody-minded mutineers deserted poor old Captain Bligh, they fled and became the first Brits to settle with the Pitcairns’ Tahitian inhabitants.

They then feuded on a scale that makes the Labour party seem like a gentle brotherhood. Eleven years ago, six of their male descendants returned to the headlines accused of sexually abusing under-age children. Now, in the coalition’s final hours, the marine reserve is offering them a treasury of fish and plant life that tourists and scientists will pay to visit. Overfishing has killed off most of this richness in other waters on the planet.

In this introverted election campaign, nobody began by reminding us that Britannia still rules a surface area of waves 27 times greater than its land mass. Our seas are not only punctuated by tax havens and the brass plates of absentee company headquarters. Under them cruise our imperilled tuna fish, masses of swimming species, many of which are endangered or as yet unnamed, and rare types of non-financial shark. There are also the world’s most beautiful gardens, made of still-healthy coral, and hundreds of species of algae, even more photogenic than the roses at Chelsea Flower Show.

On land, Britain’s gardeners have been made to feel it is for them to “save the planet”. Compared with farmers, their contribution can only be minimal. Compared with fishermen, it is negligible. Far more of the plant life in Britain’s territory is underwater than above it. The new marine area to protect it was not mentioned by the Chancellor in his Commons speech. Perhaps there were fears of a mutiny in the Budget. The statement’s small print delivered his blue-green bombshell, doing more for endangered wildlife than the Green party had so far managed to promise over cups of cowslip tea.

The blackside hawkfish

In its dying days, the last Labour government trumpeted a big, protected area around the Chagos Archipelago in the Indian Ocean. Last summer, President Obama announced a marine haven around the Pacific Remote Islands — bigger, just, than the Pitcairn plan, but not such a continuous area of protected water. In their manifesto the Tories propose to surpass it by adding another 80,000 sq miles of British sea territory as a reserve around Ascension Island and committing to 14 blue belts around our overseas territories. There will also be 127 marine conservation zones around the English coastline. The plans are not those of a party electorally at sea. They have been consulted and thought through. They give a new edge to “joined-up government” and are the most ecologically friendly legislation from any party in British history.

It has been rotten to be a university student under the coalition, but under the Tories it will be great to be a green sea turtle.

The titan triggerfish

Behind the bold idea are charities and celebrity-advocates. In February this year, Helena Bonham Carter posed naked with a bigeye tuna to publicise the case for sea reserves in overseas territories. On hearing the announcement she added good news of her own, that she “never knew that taking my clothes off could be so effective. I must do it more often.” The Great British Oceans’ coalition of charities and pressure groups, ranging from the National Geographic Society to the RSPB and The Pew Charitable Trusts has been instrumental, but Downing Street gives special credit to the UK’s Blue Marine Foundation.

During the plan’s adoption there was little enthusiasm in the civil service, fearing practical failure. So I asked Blue Marine’s chairman, the experienced ecological commentator Charles Clover, if he thinks it will really work.

A postage stamp from the Pitcairn Islands

He is entirely confident about the Pitcairn area. Local agreement was already forthcoming, he points out, whereas the Chagos announcement provoked protests due to the inhabitants’ eviction from the islands in the 1970s. The 56 remaining inhabitants on the Pitcairns are not going to be offered another such global opportunity again. “If we can force British overseas territories to hand over details of tax cheats,” Clover reminds the plan’s sceptics, “we can surely agree they will do their absolute best to police a marine reserve.” Foreign Office advisers have been fearing the expense, as if naval task forces will have to be deployed. The more their fish breed, the more of a bait they will become to the next predator in the food chain, pirates with nets and guns.

Poster for the 1935 film ‘Mutiny on the Bounty’, starring Clark Gable

Clover is clear that the cost of policing the Budget’s Pitcairn plan can be capped at not more than £500,000. The reason is the new scope for digital policing. If a rogue Japanese trawler tries to let down its nets in a reserve, it will be caught on the digital record of newly devised satellite devices, which will be monitoring local traffic. Huge fishing fines will be dished out. The offending ship will then be blacklisted and excluded from the world’s fishing grounds. It is a use of digital techniques that not even my daughter, Baroness Lane-Fox, found time to mention in a recent BBC Dimbleby Lecture on our changing digital future.

Picture of the house on Pitcairn Island belonging to Fletcher Christian (1764-93), leader of the Mutiny on the Bounty

The prizes are high: not only a fast-breeding ground for underwater plant life and fish, but the naming of a commemorative species for the scheme’s big backers. Clover credits crucial support for his charity’s plans to Jo Johnson, brother of Boris, former Lex journalist at the FT and now head of Downing Street’s policy unit. “It is a coup,” Johnson told me, “and it will work. We now need to choose and name a species after George Osborne.”

It has been rotten to be a student under the coalition, but under the Tories it will be great to be a sea turtle

I have no difficulty in naming a species for Ed Balls. If he pays for the Tory manifesto plan for Ascension Island, he can give his name to a species of their black triggerfish, the most aggressive predator in their waters. Osborne is more problematic, but Enric Sala, explorer-in-residence at National Geographic, has guided me.

“Why not name one of our newfound basslet fishes as Pseudanthias osbornei?” Sala has seen it below 200 metres. I like the idea of a chancellor bottom-feeding as a pseud in dark pools. Watch out, though. Its popular name is “sea goldie” and in times of shortage, slimmer species are not safe from a goldie, either.

Photographs: Enric Sala/National Geographic; Alamy