Does the UK have 'bigger fish to fry' in France row?

"We have much bigger fish to fry."

This was Boris Johnson's attempt to crack a joke about the spat between the UK and France over fishing rights in the Channel came, at the same time as he is trying to twist world leaders' arms to make bigger, bolder commitments to help slow down climate change.

He's not doing so just because of his own conviction the most powerful countries in the world are running out of time to make a real difference to the real threats that future generations face from changes to the climate.

But progress is vital if he is to make a political success of the next fortnight, when he is playing host to nearly every country in the world at the UN environment conference in Glasgow, which now starts within hours.

The prime minister's team says he has no wish to crank up hostilities between the UK and our nearest neighbour. Yet the UK is increasingly fed up with what it considers now a pattern of behaviour from the French government in the dispute over fishing rights.

Government sources talk of frustration with French rhetoric, and French threats, to increase extra customs checks on Channel trade, to refuse British boats access to French ports, or dangling the possibility of disrupting the energy supply to Jersey.

And, as one insider suggest, the UK government increasingly believes that its French counterpart is "not acting in good faith".

But French frustrations have been rising too, with a deadline set for Tuesday for the UK to revise its stance on fishing permits. And across the EU there's also been irritation with how the UK is trying to rejig the other part of the legal agreement it signed with Brussels, remember all the cross words over the arrangements for Northern Ireland.

Some point to the symmetrical frustration - the EU's annoyed with the UK interpretation of that protocol. The UK is irritated with how the France is treating the arrangements that cover fishing permits.

If the hypothetical French threats turn into real action which might break the legal terms of the trade deal with the EU, Boris Johnson's Brexit lieutenant, Lord Frost, made plain on social media today that legal action against the whole bloc is an option.

At the very least it's a diplomatic distraction. At worst it could tie up the UK and the EU in months of legal wrangling.

It's not however yet at the stage of the UK and France screaming, 'see you in court'. If the UK triggered the 'dispute settlement mechanism', there would first be a series of high level talks to try sort out the problem, and if those failed, both sides would appoint their own lawyers who would then try to work out who is in the wrong.

Interestingly this would not be a process covered by the European Court, but a test of process the two sides sweated over last year.

There are opportunities for both sides to pull back. Boris Johnson and Emmanuel Macron will meet one on one tomorrow. The prime minister appealed to the boss of the EU council, Ursula von der Leyen to help sort it out today. But even if in the coming days the two sides pull back this time, there's an air of resignation among some in government about the politics at play here too.

There do seem to be practical problems about fishing rights in the Channel. But President Macron faces an election in the spring, and one insider suggest until that contest has been concluded there'll be an inevitable "ebb and flow" in verbal diplomatic complaints about the UK.

What's not clear though is whether the rhetorical arguments around the spat will turn into something more serious and real. The tension maybe dwarfed by the gravity of the climate challenge, and Boris Johnson's determination to create momentum.

But the argument with France is not something that he can, or intends, just to ignore.