Do UK elections accept write-in candidate votes?

i don't believe so.

Certainly not.  What would be the point?  And there is no space provided on the paper for it.

Anything other than one cross that clearly indicates one candidate (or two or three if it's a local election where you can elect two or three councillors for your ward) is counted as a spoilt paper.  A tick rather than a cross will be accepted.

The exception is Scottish local elections, which use STV so you're expected to number the paper in order of preference.  But again, what you meant must be clear.

Spoilt papers are even counted and announced according to the reason - "want of official mark", "voting for more candidates than specified", "unmarked or void for uncertainty", and my favourite one, "mark by which the voter can be identified".  Apparently some people haven't twigged yet that it's a SECRET ballot and they're not supposed to sign the paper.

Still, it's a fine old tradition to spoil the paper if you consider there is nobody worth voting for, and as spoilt papers are still counted, it achieves much the same as a write-in candidate and shows you did at least turn up.

No, that's a US thing not a UK one.

Put it this way if the UK had a write in option then most people would vote for Votey Mcvoteface

The US is pretty much the only country in the world where this is allowed. If you don't make a clear mark (a tick or X) in one box (occasionally more than one, but it is always made clear if this is the case) then the ballot is spoiled.