Can a political party unilaterally change candidates in the run up to a general election campaign?

For example if too many MPs support policy X and this is seen as an electoral risk, can the party simply change candidate to find someone more “liked” by the electorate to have a better chance of winning?

There is no law preventing it

To become an MP representing a main political party a candidate must be authorised to do so by the party's nominating officer.

Parties are free to choose candidates by their own means in the UK. They are governed by their own internal procedures, which vary by party. The process of an incumbent candidate being refused the nomination is known as deselection (and has been a controversial issue lately, especially in the Labour party.)

This is generally carried out at the local level, with a constituency party deciding who is nominated as their candidate. That said, a party could write their rules such that it was done centrally by the party leadership.

Yes. It is called "deselection".

Parties can stand whatever candidate they like in each constituency. It is empirically the case that the previous incumbent, running as the same parties' candidate, is more likely to win, than a replacement, so the normal practice is to run the incumbent again if they desire.

Having been deselected against ones will, it is perfectly possible to run against the other selected candidate. This will either be as an independent, or for a different party. This does have the problem of splitting the vote, and often means someone else wins.

Threats of this kind are currently being used by constituency Labour parties¹, to align "Blarite" and "Brownite" MPs to become more "Corbynite". This pressure was named as a reason why some of MPs left Labour (to form "The Independant Group" with some ex-Conservatives)

Also currently facing deselection is the Conservative MP Nick Boles, for being too "Remain".

¹ Under Labour rules, local party members get much of the choice of whom to stand as the official Labour candidate.

To become nominated as a candidate at a UK Parliamentary general election in Great Britain, you need to submit a completed set of nomination papers to the place fixed by the (Acting) Returning Officer by 4pm on the 19th working day before the poll. This deadline is set out in law and cannot be changed for any reason

From the Electoral Commission. This is true of parties and independents.

Up until that deadline, the parties are free to operate according to their own rules on candidate selection. After that deadline, the candidates are locked in. I believe there have been cases where someone died between nomination and the election, and remained as the candidate and even got votes.

Obviously the candidate selection doesn't change the incumbent, who remains MP until after the election.