This is easier to detect than you might think.
Each polling station has a Register of Electors at it. That list is crossed as each person votes. once polling ends, that list is shipped back to the council, who sell it to each of the parties. Thus, the parties have a full list of who has voted where. I believe that the same process works with Postal votes too.
So, if you are registered in two locations, it should be fairly easy to spot people voting twice. It is, of course, much easier to pick up people with unusual names, whereas it's harder to spot the John Smiths of the world.
I don't know how much of the verification data (eg NINOs that they also get).