Does Theresa May have the power to biff Boris?

Has Boris Johnson tried upstaging Theresa May? Credit: PA

At a time of acute global insecurity, and on the eve of the 72nd session United Nations General Assembly in New York, it is a marvellous look for the UK that its Foreign Secretary Boris Johnson is rowing with the country's chief statistician Sir David Norgrove about whether he told a porky when repeating the claim that leaving the European Union will liberate £350m a week for deployment in the NHS.

As for the Prime Minister, with whom I and other hacks are travelling first to Canada and then to the UN - to discuss crises from North Korea to Burma to the global threat of terror from so-called Islamic State - she will be in two minds about how Johnson's attempt to upstage her Brexit speech, planned for Friday, has instead imprisoned him in a fatuous spat he cannot win (unless he accepts that the £350m is not a robust number, but simply symbolises the unwillingness of Brexiteering ultras to pay anything to the EU for market access after the UK leaves).

On the one hand, Johnson has succeeded in isolating himself, with what looks like a disloyal attempt to destabilise her, from most of the Cabinet and her backbench MPs.

That is good for her.

On the other, her apparent inability to punish him merely underlines her own tenuous grip on office.

The prime minister is off to see Justin Trudeau in Canada. Credit: PA

One of the benefits of Brexit was supposed to be to show a renewed and confident UK setting the global foreign policy agenda.

It is obviously a teething problem, but right now Brexit instead is simply reminding the world how adept we are at making soap operas.

P.S. The centrepiece of May's meeting with the PM of Canada, Trudeau, will be an understanding that Brexited Britain will try to replicate and build on Canada's newly negotiated trade deal with the EU.

Which would be a triumph. But of course will also be a reminder that the UK would not need to replicate that trade deal were it not leaving the EU.

The point is that for now and some time, Brexit means not enormous freedom to secure trade deals not available to the EU, but the challenge of not suffering worse terms of trade with the rest of the world than the 27 EU countries to whom we are bidding farewell.