Boris Johnson will challenge Donald Trump to throw open US markets to British exporters after Brexit, when the two men sit down together on Sunday for their first face-to-face meeting since Johnson became prime minister.

Speaking to reporters en route to the G7 summit in the French town of Biarritz, Johnson listed a string of British products, from cars to cauliflowers and pork pies to rulers, which he claimed faced unnecessary export barriers in the US.

“You may not know this, but there are currently restrictions on the sale of British-made shower trays to the US. We’ve sold 250,000 shower trays around the world. There is some kind of bureaucratic obstacle that stops us selling them in the US because they are allegedly too low,” he said.

The comments echoed Johnson’s use of a kipper in the Tory leadership campaign to bemoan EU bureaucracy – though the rule in question subsequently turned out to be a British one.

“There are tariffs on cars, there are tariffs on railway carriages in the US of 14%, whereas we only have a tariff of 1.7% in the UK on American railway carriages arriving here,” Johnson said.

“It’s not just beef or lamb that is currently banned from entering the US in spite of their commitment to overturn that prohibition in 2014. Not a morsel of British beef has entered the US market,” he added.

After setting out a long list of products and sectors, he said: “The point I am making is that there are massive opportunities for UK companies to open up, to prise open the American market. We intend to seize those opportunities but they are going to require our American friends to compromise and to open up their approach because currently there are too many restrictions.”

Johnson declined to set out what the UK can offer the author of The Art of the Deal in exchange for these opportunities, but he stressed that the NHS would be “completely off limits” in any post-Brexit trade deal. “We will not allow the NHS to be on the table at all,” he said. However, he did express a willingness to consider redrafting the UK’s proposed digital services tax, to be levied on US giants such as Google.

“Frankly, we must do something to tax fairly and properly the online businesses that have such colossal sales in our country,” he said, but added: “I am open to discussion about how we do that and I am willing to listen to our American friends.”

Cross-Whitehall research published last year suggested a new free- trade deal with the US would add 0.2% to GDP over the long term.

The most important thing for any prime minister of the UK is to have a very close, friendly relationship with our most important ally Boris Johnson

The G7 summit is Johnson’s debut on the international stage as prime minister. He hopes to underline Britain’s determination to remain outward-looking and multilateralist after Brexit – and try to burnish his credentials as a statesman.

The US president has called the PM “Britain Trump”, and the pair struck up a rapport when Johnson was foreign secretary, but Downing Street hopes to avoid any impression that the PM will be a patsy.

Asked whether he agreed that he resembled the US president, Johnson said: “I was born in the United States. I think the most important thing for any prime minister of the UK is to have a very close friendly relationship with our most important ally and that’s what I intend to promote.”

But he sought to distance himself from Trump’s trade policies, calling for a dialling-down of the dispute between Washington and Beijing.

Johnson said he was “very, very concerned,” about the tit-for-tat conflict between the US and China. The two countries have imposed tariffs on a swathe of each other’s imports, as the White House accuses Beijing of unfair competition and China retaliates. On Friday, Trump issued a series of bizarre tweets, including one “ordering” US firms to divest from China, after Beijing announced new retaliatory measures.

Johnson said: “This is not the way to proceed. Apart from anything else, those who support the tariffs are at risk of incurring the blame for the downturn in the global economy, irrespective of whether that is true.

“I want to see an opening up of global trade, I want to see a dialling down of tensions, and I want to see tariffs come off.”