Contrary to popular myth, Britain does have a constitution. Unlike the American one, it’s not written down in one place. But Britain has Magna Carta, the 1628 Petition of Right, and the 1689 Bill of Rights. It also has centuries of precedent governing how business in Parliament is to be done.

And Brexit’s opponents are trashing that constitution.

Joining the European Union was the first assault on Britain’s constitution. It meant that, by vote of Parliament, Parliament itself was no longer supreme. In days gone by, Parliament had installed and executed monarchs, funded wars and given the working class and women the vote.

But after 1973, and increasingly thereafter, Parliament’s ability to make the rules that govern Britain shrank steadily. Having been brought up on a tradition of powerful and representative parliamentary government, the British people were always uncomfortable with this new reality.

That, more than anything else, was what shaped the outcome of the Brexit Referendum in 2016. As Sir Michael Caine put in in 2017, he voted for Brexit because he would rather be a “poor master than a rich servant.”

By 2016, the referendum itself was a well-established part of the British system. Every major party had promised, or even demanded, a referendum on Britain’s EU membership over the past decade.

Prior to 2016, there had been referenda on membership of the EU’s predecessor in 1975, Scottish and Welsh local government, Britain’s voting system, on whether northern England should have a regional assembly, and on Scottish independence.

These referenda were hard-fought, and their results were always respected.

Not so the Brexit Referendum. Despite Leave’s victory and the fact that in the election of 2017, 85 percent of the British public voted for a party that explicitly pledged to back Brexit, a near-majority of MPs still hanker to remain in the EU’s customs union.

To achieve this, they have tried every trick in the book. They have sued to frustrate the result of the referendum — and the courts have complied. The British civil service is supposed to be non-partisan, but anonymous civil servants repeatedly complain that their colleagues are on side with Remain.

After the House of Commons refused to pass Prime Minister Theresa May’s terrible deal (one that would ensure Britain keeps taking its rules from Brussels), Mrs. May and the EU changed Brexit’s effective date from March 29 to April 12.

Like the man with the broom following the elephants, the Commons was called in afterward to validate this change — but only to make British law comply with what was now purportedly international law.

The implication is that, no matter how often the Commons votes to leave the EU, it can simply be overridden by the EU’s supposedly higher power. This makes a mockery of EU’s own rules — but the EUis only interested in following its rules when they work in Brussels’ favor.

At the core of the British Constitution is the relationship between the Commons and the government. The Cabinet is governed by “collective responsibility,” meaning the policy the Cabinet adopts is binding on all of it. Members of Parliament, in turn, question the government and can become ministers.

But with Remain at the helm, collective responsibility has broken down, with ministers repeatedly voting against the government. So many of them have resigned that entire government departments are vacant. Nor do new potential ministers want to sign on to a government that smells like death.

Speaker of the House John Bercow is supposed to be non-partisan. But while his decisions have randomly favored first Remain and then Brexit, most of his throws of the dice have come up EU-blue.

Mr. Bercow’s justification? If Parliament was guided only by precedent, then “nothing ever would change.” Nonsense. There is clearly a way to change the British system: Win an election, command a majority in the Commons and vote your policy through. Bringing change to a game isn’t the referee’s job.

But the worst of Remain’s follies was the endorsement by the Commons of a Remainder motion last week to take control of the Parliamentary timetable.

This is a terrible departure from precedent. As Professor Vernon Bogdanor, an authority on the British Constitution, noted, it would be “unconstitutional for parliament to take over the Commons timetable to impose an alternative form of Brexit.”

The Commons can either support the government or change it: It cannot do the government’s job for it. In the end, the votes after the Remain motion came to nothing. But it throws every future British treaty and spending bill into doubt by asserting that the Commons is a co-equal actor with the government.

Brexit won in 2016 because it embodied a popular sense that Britain needed to take back control. The Remain establishment hates this. They have a clear goal: Keep Britain in the EU.

And then sit back and applaud as the EU trashes the British Constitution some more.