Since there is a formal deadline for withdrawal of candidacies from UK Parliament elections, are there historical examples in which some candidates nonetheless informally withdrew after such a deadline had passed, by unequivocally telling their [potential] voters not to vote for them?
One example that springs to mind (mainly for its strangeness) is that Willie Rushton (of Private Eye) ran as a protest candidate against Alec Douglas-Home, the PM, in the 1963 Kinross and Western Perthshire by-election - he recommended his supporters vote Liberal. I am fairly sure there are a number of similar examples, though most are probably minor parties or independents rather than a major-party candidate having a massive change of heart.
(An inactive paper candidacy is not really the same thing, as there is still an expectation you will take all the votes you can get. It wouldn't be a paper candidacy if you endorsed someone else)
Once a person is nominated to stand for election within a constituency there is no way to remove that person from the ballot paper. Returning Officer Guidance 2015
The details of any candidates who by the close of withdrawals remain validly nominated must be printed on the ballot papers. If a candidate has not withdrawn by the deadline, their name must go forward to the ballot paper.
However, there is no obligation for the candidate to canvas for support or do anything else really in relation to the upcoming vote. A candidate that doesnt look for support is known as a Paper Candidate
This was suggested to be the ploy of the Conservative Party in some current Labour seats in the upcoming UK General Election
The prime minister was said to be willing to put up "paper candidates" in Labour marginals, with the Tories carrying out only minimal campaigning in order to give an advantage to their Brexit Party rivals.