Britain's Brexit negotiators behaved "like idiots", a senior EU figure has said, amid both comical and "terrifying" scenes in London.

The self-admittedly "harsh" language came from Dutch European Commission vice-president Frans Timmermans in a BBC interview recorded earlier this year, which is to be broadcast on Thursday (18 July).

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Boris Johnson's ally claimed Britain would be £80bn better off in a no-deal exit (Photo: UK Foreign and Commonwealth Office)

"I saw him [former British Brexit negotiator David Davis] not coming, not negotiating, grandstanding elsewhere [and] I thought, 'Oh my God, they haven't got a plan'," Timmermans said.

"It's like Lance Corporal Jones, you know, 'Don't panic, don't panic!' Running around like idiots," the senior EU commissioner added, referring to a British TV comedy character.

Britain's likely new prime minister, Boris Johnson, also came in for criticism.

"I have always had the impression he is playing games," Timmermans said.

Laughter did ripple through the audience in London this week when Johnson held up a smoked fish at a political debate.

"If you want to understand why it is we must leave the EU ... I want you to consider this kipper," he said.

EU rules meant the fish must be shipped with an "ice pillow", Johnson claimed, in what "massively increased" costs due to "Brussels bureaucrats".

"Ladies and gentlemen, when we come out, we'll be able to ... end this damaging regulatory overkill", he said.

"We need to get on and get out of the EU on 31 October," he added.

Johnson is odds-on to beat his only rival, Jeremy Hunt, and become Britain's new leader in a vote in the ruling Conservative party on 22 July.

And it was "pure silliness" to think the British economy might tank if Britain left the EU with no deal, a Johnson ally, Tory MP Jacob Rees-Mogg, also said this week.

The UK would be £80bn (€89bn) better off if it just left, Rees-Mogg claimed.

Funny?

"Terrifying that someone this close to a potential future government can think we'd actually be better off by adding barriers to access to our largest market," the British finance minister, Philip Hammond, retorted on social media.

The British economy would slip into recession in 2020 and shrink by three percent in a no-deal exit, British national forecaster, the Office for Budget Responsibility, also said on Thursday, making mockery of Rees-Mogg's maths.

A "harder Brexit" could see the British pound hit parity with the US dollar in its lowest fall since the 1980s, US investment bank Morgan Stanley added.

For all their comic value, the trolling and populism come in a political scene which has "gone down the wrong path", the outgoing British prime minister, Theresa May, warned in a speech on Wednesday.

It was a scene in which "one believes that if you simply assert your view loud enough and long enough you will get your way in the end," she said.

There was "a very ugly form of nationalism developing" in England, David Melding, a Welsh MP added, which "turns any failure on the part of the UK government to somehow be the fault of Johnnie European foreigner".

Michel Barnier, the EU's Brexit negotiator, also sounded serious when he told the BBC documentary the EU was better prepared than the UK for a no-deal scenario.

"We've never been impressed by such a threat. It's not useful to use it", Barnier said on Johnson's main pledge - to leave the EU on 31 October no matter what.

Selmayr's deal

The BBC, which shared teasers of its show with other media, gave behind-the-scenes peeks of the EU talks.

At one point, Martin Selmayr, the EU commission's top civil servant, told David Lidington, a senior British minister, the UK should freeze Brexit for five years and do a new deal to stay in, the British broadcaster reported.

"Martin sort of said: 'Look, why don't we have a deal whereby we just put all this on ice for five years? Let's see how things go, let's get the UK involved with France and Germany, let's see how the dust settles and let's talk about whether we can come to a new deal for Europe'," Lidington told the BBC.