Yes, I raised this with the staff at the polling station. There is NOTHING to stop me rocking up first thing, vote using my neighbour's address, watch to see if the staff have changed in the evening, and then going again to vote using my address. I don't even have to take the polling card with me. truly amazing. Nice to have a system that works on trust, I suppose, and what I am describing is presumably rare (?) but why be loose about security, when at least proof of ID, even it is a Utility Bill in your name, is easy to bring along and is surely at the very least better than nothing.
The staff won’t have changed as they are there for the duration. I doubt they would notice you had been in earlier, especially on a busy day, but what about your neighbour when he turned up to vote and was told he had already voted?