There can’t be a more successful tourism marketing campaign than “100% Pure New Zealand”. And New Zealand is seen as a world leader in another respect: how its farmers sell their food globally without government subsidies. But the tension between these two successes has been exposed by Sir Tim Smit, the co-founder of the Eden Project in Cornwall.

Smit has sparked a minor international incident by declaring that New Zealand is “so pure the people of Christchurch won’t even swim in the river Avon. Most of the lakes are full of algae. It is like a beautiful person with cancer.” He was speaking to British landowners pondering the New Zealand model for post-Brexit agriculture. British farmers currently receive £3bn in subsidies each year; environment secretary Michael Gove must design a much smaller subsidy system – or scrap it altogether.

Smit argues that New Zealand is no inspiration despite its farmers surviving the removal of subsidies in 1984. Their response has been to intensify, and export milk to China.

Landscapes that George Monbiot might describe as sheep-wrecked are now cattle-wrecked; the cost, argues Smit, is water quality, as nitrates from fertilisers flood into rivers. Smit’s critique has been endorsed not only by Greenpeace New Zealand but also by the chief executive of New Zealand’s fish and game council, Martin Taylor, who calls the country’s clean, green image “a facade”. Brexit marks a crossroads for the British countryside. Gove believes we can boost agricultural productivity while also enhancing the environment. His vision isn’t so dissimilar from some environmentalists’: subsidy-dependent upland farming will cease (and some land even rewilded) while production is intensified on surviving lowland farms.

But the idea that we can have it all – cheap food, pristine environments and profitable farmers – is naive. Without tough regulation and subsidies for farmers to provide public goods such as healthy soil, clean water and carbon storage, we will all, ultimately, suffer.

No accounting for nature

I suggested the Isle of Man could imitate New Zealand’s “100% Pure” marketing when I visited last week. It was an answer to questions from islanders frustrated that their public image is of 83,000 accountants clinging to a low-tax rock in the Irish sea.

I’m always struck by Man’s deliciously clean air, its variety of scenery, from moors to mountains to luminous green pasture, and the fact that the sea is always unexpectedly close. With superb marine nature reserves, the island offers excellent whale-watching; when its skies are clear (not often, in my limited experience) its lack of light pollution makes for superb star-gazing.

Clean air, healthy seas, stars – these are our true riches. Unfortunately, it looks like post-Brexit Britain will seek to emulate Man’s financial arrangements rather than its natural wealth.

A mammoth mistake

On Sunday a new BBC documentary will tell the tragic tale of Jumbo, the 19th-century circus elephant who gave his name to big jets. When Jumbo died in a collision with a train in Canada, his owner claimed Jumbo saved his keeper’s life by deliberately facing down the train. In fact, scientists examining his skeleton have discovered, Jumbo was hit from behind: the maltreated elephant was running away.

We may feel good that Britain’s last circus elephant was taken into a sanctuary six years ago. But given elephants’ size and what science is telling us about their sensitivity and intelligence, we shouldn’t be keeping them in zoos at all. I’ve no doubt that a 22nd-century documentary will upbraid us for that.

• Patrick Barkham is a Guardian columnist