I ended up (overcautiously, I know) cancelling the postal vote and appointing a proxy in the end "just to be sure" - probably just being paranoid about the Royal Mail.
But the following day, the (now cancelled) postal vote then arrived!
Thanks for the replies though.
Thanks for update
actually that is typical bad luck - but you probably need to be 100% sure that the PV is cancelled to prevent the risk that your proxy turns up to vote on your behalf on election day and gets refused the vote on the basis that records show you had a postal vote...
So I would say:
I think what will happen now is that you and your Proxy will be issued letters from the council (or maybe e-mails) confirming the proxy - make sure your proxy has these (including a copy of the one sent to you perhaps) and takes them with them when they go to vote. I would
not destroy the postal vote either (you could potentially fill it in,
seal it, and give it to your proxy to take to the polling station to give to them in the event of the staff refusing to issue the proxy on the day on the basis that you were supposed to have voted by post). If the proxy can vote OK, your proxy should just give you back the un-deposited postal vote for you to destroy when you return from holiday -
so you can proove you have not tried to vote twice. If you or the proxy send in the postal vote the ballot ref number on the signature declaration part of the postal vote docs should show to the council staff that this is in fact a cancelled postal vote and they would discard it
I realise I am probably being unnecessarily over the top in my advice, but this is the way to try to prevent your proxy being turned away on the day and not being able to do anything about it because you are out of the country.
Assuming the rules have not changed fundamentally I would think in the event of any such problem / dispute at the polling station the presiding officer should issue your proxy with a different coloured ballot paper (not sure what these are called) to allow your proxy to vote on - they keep supplies of these which are used but only counted in the event of the election being so close that the different coloured ballot papers could swing the election outcome. Since voter ID I suspect they are less common or may have been abolished even - but they were / are used in the event of someone coming to vote and finding that someone had already voted using their name and address earlier in the day (ie committed electoral fraud) - such that the fraud did not deny the true voter the chance to vote. If the suspected fraud affected the outcome of the election then I think that ultimate result of that would be the election in the constituency concerned being re-run. I think this may have actually happened in one constituency in 1997 for some reason such as this tho I've not double checked that.
There are probably some up to date presiding officers on this Forum who can correct me if any of the above is wrong / bad advice !!