@Chersfrozenface The voting paper should be in a separate sealed envelope so if your ID form is invalid, then the vote envelope is set aside still unopened.
You are informed that your ID form was invalid without anyone seeing your vote as it's still sealed. All voting papers are kept as you say, including those unopened ones. But those should remain unread, to my knowledge. That's why there are different categories for invalid as mentioned earlier in the thread.
In theory, I suppose, someone could open both your ID form and your ballot paper at the same time, but my view is that may well fall under ballot tampering and potentially be a criminal offence. Possibly, under s.65 of the Representation of the People Act 1983
It's the ID ONLY that's checked, NOT what is in the separate sealed ballot envelope accompanying it.
I suppose if you got a letter saying that your vote was void for ID and you got in touch with them saying "I never sent in a postal ballot" then there would be a criminal investigation. I'm not sure what happens at that point. as I've no experience of it.
No system is a hundred per cent foolproof I know. If you are interested, here is what Rochdale's site says.
www.rochdale.gov.uk/elections-voting/postal-voting/3
"Postal voting safety and fraud
Voting by post is safe and proven cases of electoral fraud are rare.
When voting by post, you should mark your vote on the ballot paper in secret, and seal the envelope yourself.
Verifying your identity when you vote by post
When you apply for a postal or proxy vote, you'll be asked to give your date of birth and signature. When you return your postal voting pack, these details are checked to confirm your identity, which makes postal voting safe.
It’s important that you read the postal voting instructions carefully before you vote by post, and provide the right information in the right place.
Half of all postal ballots rejected at the last UK general election had a signature or date of birth that did not match that given in the initial application. One common mistake people make when voting by post is to provide the date of completion, rather than their date of birth.
Your signature and date of birth are separated from your ballot paper before its looked at or counted, so giving this information will not affect the secrecy of your vote."