In December 2015 the British Government, the Mayor of London, and the Metropolitan [London] Police all rejected the claim. Downing Street said the claims were "totally inaccurate", the then Mayor of London Boris Johnson said they were "utter nonsense", and the Met said:
"We would not normally dignify such comments with a response, however,
on this occasion we think it’s important to state to Londoners that Mr
Trump could not be more wrong... Any candidate for the presidential
election in the United States of America is welcome to receive a
briefing from the Met police on the reality of policing London."
The Prime Minister's spokesperson said:
"The Prime Minister completely disagrees with the comments made by
Donald Trump, which are divisive, unhelpful and quite simply wrong,"
Boris Johnson went on to say:
"As a city where more than 300 languages are spoken, London has a
proud history of tolerance and diversity and to suggest there are
areas where police officers cannot go because of radicalisation is
simply ridiculous... Crime has been falling steadily both in London
and in New York - the only reason I wouldn't go to some parts of New
York is the real risk of meeting Donald Trump"
Also worth noting that at the time the government, prime minister, and mayor of London were all Conservative. So their reaction can hardly be dismissed as leftist conspiracy or such.
If there's any parts of the country where the police can't go owing to radicalisation, it's likely only some parts of Northern Ireland. See The Troubles.