The US President added that he had told European leaders that immigration is a problem.
For no apparent reason, he chose to go on the attack against Prime Minister Theresa May, London Mayor Sadiq Khan, and the nation's immigrants - showing that his bigotry crosses borders.
In a statement ahead of Trump's arrival, she said the visit would focus on trade and strengthening defence and security ties, saying there was no stronger alliance than Britain's "special relationship with the USA".
British pageantry was on grand display Thursday as President Donald Trump and first lady Melania Trump arrived on the historic grounds of Blenheim Palace for an exclusive black tie gala hosted by Prime Minister Theresa May.
Earlier in a hastily arranged press conference before flying from the North Atlantic Treaty Organisation summit in Brussels to Britain, Mr Trump told the Prime Minister that she is not delivering the Brexit the people voted for. Some of their signs read: "Dump Trump", "Lock Him Up", and "There Will Be Hell Toupee". You see what's going on throughout the world with immigration. But on Thursday they said they welcomed the plan and would study it closely.More news: North Korea fails to attend talks on repatriation of United States war dead
May's "business-friendly" Brexit plan - which would keep Britain in a free trade zone for goods with the European Union but mean it has to share some European Union rules - was agreed by her cabinet only last Friday after two years of wrangling since Britons voted to leave the bloc in a 2016 referendum. "I think that's why Brexit happened".
In the busy south London neighborhood of Brixton, Victor Greetham, who runs a jazz bar, said he hasn't been to a demonstration since he protested against the Iraq War in 2003, but he planned to attend one of the protests against Trump's visit.
Ian Blackford, the Scottish National Party's leader in the Westminster parliament, said he regretted that May's government would roll out the red carpet for Trump.
Tall metal mesh barricades and concrete blocks have been erected around the perimeter of the US Ambassador's swanky central London residence, Winfield House, where Trump will spend Thursday night.
Khan gave protesters permission to fly the 20-foot-tall balloon, depicting Trump as an angry baby in a diaper.More news: Facebook slapped with six-figure fine in United Kingdom over Cambridge Analytica breach
It isn't a state visit, but it began with all the pageantry that President Trump has shown such an appreciation for on previous foreign trips.
In other news from this morning's press conference, Trump stunted expectations when he refused to comment on the UK's current Brexit turmoil.
A Trump Baby balloon has been given permission to fly over central London during the USA president's visit to the United Kingdom after police in Scotland refused a request for the 6m (20ft) high balloon to also take to the skies over the golf course.
Khan said a year ago that Trump's trip to the United Kingdom should be cancelled after the president retweeted videos from a leader of an ultranationalist British party that claims to show Muslims engaging in violent acts.More news: England manager Gareth Southgate reflects on World Cup exit
And in an apparent plea to the president to remember his allies when he meets Vladimir Putin on Monday, Mrs May noted that Britain and America work closely on security "whether through targeting Daesh terrorists or standing up to Russian aggression".