If everyone on the voting list for a polling station had been to vote and have been marked off on the list the polling clerks have, would the polling station shut early? Or would it remain open until 10PM even though no one else could come in and vote?
There's a process that's meant to be followed if someone arrives at a polling station to find that someone has already voted in their name or they're recorded as having received a postal vote (a 'tendered vote' can be made, although it isn't counted).
If the polling station closed early then this might be made impossible. For this reason it would not make sense to close early.