It looks like the British Parliament is going to debate whether to ban Donald Trump from Britain. It would be crazy to ban him, but the debate also presents an opportunity for The Donald, who has yet to articulate a foreign policy on Europe.

Parliament’s Great Trump Debate is scheduled for January 18. The House of Commons was forced to address the question after enough Britons signed a petition to ban Mr. Trump in response to his call for a temporary halt on Muslims entering America.

The number of petition signers against Mr. Trump has risen to 580,000. This surge came after Prime Minister Cameron intruded into the American primary campaign by calling Mr. Trump’s remarks “divisive, stupid and wrong.”

If Trump “came to visit our country, he’d unite us all against him,” Mr. Cameron reportedly said.

Here’s how I think Mr. Trump ought to respond: He should enter the debate over whether Britain should stay in Europe. That would drive Mr. Cameron and his fellow Europhiles crazy — but that’s only one of its virtues.

Whether Britain is going to stay in Europe is the big question of the year in Britain, which is going to vote on it as soon as this summer. The right move for Mr. Trump is to back a British exit, known as the Brexit.

The Brexit is the goal of the United Kingdom Independence Party, which is a natural ally of The Donald. That party has been gaining enough ground that Cameron has just permitted the leaders of his own government to vote their conscience on a Europe referendum — meaning they’ve been released from the burden of having to toe the party line.

I’ve favored a British exit from Europe since the days of Margaret Thatcher. She gave a famous speech in 1988 in Bruges, Belgium, warning against the danger that Britain would end up being ruled by European socialists.

“We have not successfully rolled back the frontiers of the state in Britain, only to see them re-imposed at a European level with a European super-state exercising a new dominance from Brussels,” said the Iron Lady.

Europe has only gotten more dangerous since then.

To me, the logical move for Britain is to pull out of the European Union and place its bet on building the special relationship it has historically enjoyed with America. That would be good for America, too, because the “specialness” is our shared concept of liberty.

Yet Thatcher’s successors — including Mr. Cameron — have offered nothing but flapdoodle about trying to work out better terms with the Europeans. Could Mr. Trump prove to be the politician to pick up Margaret Thatcher’s call for British sovereignty?

Certainly nobody else in the American fray seems to be interested. President Obama has actually come out in favor of Britain staying in the European Union (he himself seems to be at heart a European-type socialist).

Mr. Obama has gone so far as to warn that a vote for the Brexit would cost Britain clout in Washington. Does that mean the logic of our historic “special relationship” depends on Britain becoming a vassal of Europe?

It’s not just Mr. Obama, though. None of Mr. Trump’s GOP rivals has addressed Britain’s future. No one has offered to our floundering ally the hope of a countervailing alliance with, say, America, Canada, Australia, India and Israel.

Where’s the leadership? Where’s the imagination?

What an opportunity for Mr. Trump. It’s more than just the chance to respond to the possibility of being banned in Britain. It’s a door through which he could enter the whole geopolitical debate.

It’s one thing, after all, to admire President Putin and vow, as Mr. Trump did in a recent CBS broadcast, that he would “get along very well with him.” It’s another thing to lay out a strategy to ally traditional freedom-loving nations with new rising powers.

The Donald has already written — as far back as 2000 — in favor of an American pull-back from Europe. He made that mistake because he was worried about the cost of keeping America’s forces on the continent.

Far better for Mr. Trump to put together a true foreign policy. It’s a natural issue for this election, particularly given the hash of international affairs that has been made by Mr. Obama and his first secretary of state, Hillary Clinton.

So the poser in the Parliament on whether to ban Mr. Trump from Britain is a good place for The Donald to start — and to start some constructive trouble while he’s at it. It’s not that I agree with him on a temporary ban on Muslims. It’s just that opportunities for a foreign-policy debate don’t often knock this loudly.

This column first appeared in the New York Post.