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you have a card that arrives in the post that you are meant to show when you vote
No, it's just a confirmation that you are registered and tells you where the polling station is. Edit: I suppose you are encouraged to take it with you, but that just speeds up ticking you off the register.
I don't take the card with me, I just remember the number on it (since the list they check is ordered by that number not by name or address). This speeds up everything, plus it helps me shrug at the tellers outside who try and collect the cards (so they can make and sell lists of people who are active voters - not all of them are doing this).
Voting in the UK isn't anonymous at first, but the records linking voter number and unique ballot paper number are supposed to be destroyed once they are happy that there has been no fraud at that polling station.
Nope, you have a card that arrives in the post that you are meant to show when you vote. But most of the time that’s not required.
Voter fraud in U.K. elections has not been a problem in living memory.