Nigel Farage has said private health companies should “relieve the burden” on the NHS, the UK ought to limit efforts to curb global warming, and defended anti-immigrant posters, as the rapid rise of his Brexit party led to his personal beliefs being scrutinised.

In an often testy BBC interview, Farage also tried to justify his decision to shift from praising Norway-type deals before the EU referendum to advocating a no-deal departure, saying this was needed because Theresa May had botched the process.

A poll ahead of the European elections has put the Brexit party, formed earlier this year after Farage quit Ukip, on 34% support, more than the combined total for Labour and the Conservatives.

Farage has complete policy control over his party, which has paid “supporters” rather than members, describing it in a newspaper interview on Sunday as “a company, not a political party”.

In an interview on BBC One’s The Andrew Marr Show, Farage said that while his party’s platform for the European elections was clear – push to leave without a deal – it would not have a formal manifesto, saying this had “a word association with ‘lie’”.

Asked whether previous beliefs he has expressed could be seen as a pointer to the party’s direction, Farage responded angrily, saying: “This is absolutely ludicrous. I’ve never in my life seen a more ridiculous interview than this.” At another point, he asked: “What’s wrong with the BBC?”

Farage denied he wanted to replace the NHS with private insurance, but said insurers could “take the burden off the NHS”.

He added: “If I was encouraged to opt out of the system, to relieve the burden on the health service, I would do so gleefully.” Farage said the line of questioning was “really very boring”.

Asked if he still thought measures to tackle global warming were “stupid”, he replied: “I believe that if we decide in this country to tax ourselves to the hilt, to put hundreds of thousands of people out of work in manufacturing industries, given that we produce less than 2% of global C02, that isn’t terribly intelligent.”

Farage also confirmed his belief that HIV-positive migrants should not be allowed to move to the UK, and defended the leave campaign’s anti-migrant posters in the run-up to the EU referendum, which showed a line of mainly non-white people with the slogan: “Breaking point.”

“It was the truth, and if you think about that poster, it’s transformed European politics,” Farage told Marr.

But he declined to say whether he still backed looser gun controls, saying: “This sums it up. I’ve been going round the country speaking to packed rallies every night. And do you know who’s not there? The BBC. And from this line of questioning I can see why.”

Asked why in 2016 he had no advocated a no-deal Brexit, Farage said: “Because it was obvious that we could do a free trade deal.

“The problem is the prime minister never asked for it, so we finished up in the mess that we’re in. She chose to go for this close and special partnership.

“Basically, right from the start she was happy for us to be kept very close to the customs union. So where we are now, the only way the democratic will of the people can be delivered is to leave on a WTO [World Trade Organization] deal.”

While conceding he had praised the close deals with the EU that Norway and Switzerland have, Farage insisted he had always wanted the UK to exit on very different terms.

Arguing May had “wilfully deceived us”, he said: “What she’s put to parliament three times isn’t a deal, it’s a new European treaty. I didn’t spend 25 years campaigning to leave the EU to sign up to a new treaty.”

A no-deal Brexit, he added, could bring “some short-term economic disruption – moving house leads to short-term disruption”.

Speaking later on the same show, the shadow health secretary, Jonathan Ashworth, said Farage had been evasive.

“Nigel Farage doesn’t want to stand on his record, because his record is one of wanting to privatise the NHS, it’s on big tax cuts for the very rich and penalising working people in this country,” he said.