"This will take effect next week, 5 percent", Trump said during his visit to London.
The president's unexpected compliments for May come just days before she was set to resign the leadership of her party after failing to secure a Brexit deal.
Instead, Trump called May a "tremendous professional", steered clear of rhetoric that could embarrass her and let pass contentious topics such as Huawei and Iran while making light of sensitive issues such as her possible succession.
"We'll have a very, very substantial trade deal, it'll be a very fair deal, and I think it's something we both want to do", Mr Trump told Ms May at a meeting in St James's Palace, a 500-year-old official residence of the monarch.
During an appearance on "America's Newsroom" following the news conference, Rove said Trump should have ignored Khan's criticism.
"I really predicted what was going to happen", Trump said on Tuesday.
Trump and May concluded a series of meetings with general commitments to pursue a new trade deal - although apparently without one in place.
Stephen Colbert, Trevor Noah and Conan O'Brien all weighed in on the day that Trump spent with the British royal family, which included lunch with Queen Elizabeth, tea with Prince Charles and a grand state dinner at Buckingham Palace.
Trump has already weighed in by urging Britain to walk away from the European Union without an agreement - and suggested that Brexit-backing former foreign minister Boris Johnson would be an "excellent" choice to head the government and get it done.
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The Baywatch star also wasn't bothered by criticism of her Jonas-heavy Instagram feed following their many wedding celebrations. However, they didn't go empty-handed. "If there's anyone who can handle it, it's her", Chopra insists of her friend.
Labour Party leader Jeremy Corbyn is due to address the protest, which will be attended by several senior lawmakers from his left-of-center party. Invoking the meme of a man who denied a woman a room in the Holiday Inn Express, Noah mocked Trump in the Queen's voice: "I'm sorry Donald, it's above me now ..."
Up to 250,000 people were expected to show up for the march against Trump's state visit from Trafalgar Square to the Houses of Parliament.
"That's not - I would have sued, but that's okay", Trump said, which was met with laughter.
In response, Trump heralded the British monarch, saying she "embodied the spirit of dignity, duty and patriotism that beats proudly in every British heart".
He said he didn't know May's timings but that she should stick around.
Speaking further about Brexit, the U.S. president expressed Washington's commitment to inking a "phenomenal trade deal" with the United Kingdom after it leaves the European Union.
Washington has pressured its allies around the world to bar Huawei from developing new 5G mobile networks over security concerns about Beijing's ability to exploit them for its own purposes. Huawei denies it is, or could be, a vehicle for Chinese intelligence.
The British government, sensitive about its relationship with Beijing, has been reluctant to go along with American restrictions.
He and May also joked about Trump's advice that the United Kingdom should have sued the European Union over Brexit terms. "We want to put a smile on people's faces as well as make a serious point". "We will be able to work out any differences. I don't see any protests".