To understand the Conservatives in the House of Commons who would rather topple their government than allow it to sign off on a new customs union with the EU to secure peace on the island of Ireland, it pays to go back half a century.

When Britain first expressed its intention in the early 1960s to join the European Economic Community, the forerunner to the EU, it bought about half of New Zealand’s total exports.

New Zealand’s then prime minister, Keith Holyoake, warned the former colony faced ruin if EEC tariffs were put on its butter exports to the UK, in particular.

Customs unions eliminate border tariffs and checks on the origins of goods at the border among its members, which then apply exactly the same tariffs on the imports of third countries.

Disaster for New Zealand was averted through a transition period in which diminishing quotas were sold into the UK as it joined the EEC. New Zealand started to look to elsewhere in the world to export to.

But the UK had truly pivoted to Europe, both politically and economically, and the government, while facing resistance from those romantically attached to the idea of empire, had shown scant sentimentality about severing key trade ties with the commonwealth.

Jump forward five decades, and Boris Johnson can be heard arguing that when Britain joined the common market it “betrayed our relationships with Commonwealth countries such as Australia and New Zealand”. Shortly after the 2016 referendum, Whitehall officials started talking about renewing the old ties, the so-called Empire 2.0.

A lobby group, with the support of the Commons leader, Andrea Leadsom, argued for a liberalised labour market and closer trading ties among former British dominions.

Liam Fox, the international trade secretary, whose attitude in this crunch part of the talks is being closely monitored in Brussels, recently championed UK membership of the Comprehensive and Progressive Agreement for Trans-Pacific Partnership, a trade agreement between Australia, Brunei, Canada, Chile, Japan, Malaysia, Mexico, New Zealand, Peru, Singapore and Vietnam. The British government has hired New Zealand’s former trade head Crawford Falconer as chief trade negotiation adviser.

The problem for those seeking to pivot away from Europe now is that the Irish backstop solution, in which the whole of the UK could stay in the customs union, stymies all hope of such free trade deals, and sucks the UK back in.

If a customs union for all of the UK is the “all weather” backstop, designed to be in place until a more comprehensive agreement can do the job of avoiding a hard border on the island of Ireland, it will also be in effect open-ended.

“It would mean the UK will not be able, in terms of goods, to do trade deals with third countries,” said André Sapir, a Belgian economist and senior Fellow at the Brussels-based thinktank Bruegel. “The UK trade policy would essentially, like Turkey today, have to follow EU trade policy. Not only would the UK not be able to strike deals with third countries but in addition, what is worse for the UK is that the British government wouldn’t have a seat at the table when trade deals are being negotiated by the EU with third countries such as the US.”

For the advocates of global Britain, it gets worse still. Brussels is concerned that Britain could benefit by undercutting EU rules. As a price for a union, the bloc wants the UK to commit to its state aid regime, protections on environmental standards, social and labour regulations.

While Turkey has an old-style arrangement with the bloc, more up to date agreements are designed to turn the bloc’s partners into satellites orbiting around the EU moon. “It is not Bojo’s buccaneering Britain,” joked one EU official. All eyes are now on the reaction of those diplomats in Brussels described as “Fox and friends”.