Prime Minister David Cameron has condemned Trump's remarks about Muslims as "divisive, stupid and wrong," but he and other senior officials have said they do not think Trump should be banned.
And it made me curious if this is a first, or if this is historically a thing. I've never noticed it before.
It should be clarified that David Cameron's remarks are a direct response to a motion before Parliament (triggered by petition) to ban Donald Trump from entering the UK.
So it may be a little wrong to interpret this situation as a sitting Prime Minister just reaching out, on his own initiative, to criticize a US Presidential candidate without prompting. I'm not sure if that has ever happened and I would doubt it.
That said, this whole topic is skirting close to the 20 year rule on legitimate topics, but I felt I needed to provide the context.
It is not historically "a thing" in the sense that British Prime Ministers have frequently criticised American presidential candidates. Certainly they have at times criticised or praised American presidents (preferring praise to criticism) but not candidates. So yes, this is probably the strongest and most direct criticism by a Prime Minister of a candidate (and that is how the newspapers have described it). I can't find any historical examples of it previously.
(Bear in mind that this criticism was a response to Trump's criticism of the UK: American presidential candidates have, since WW2 at least, rarely criticised Britain.)