It has been proved time and again that voting fraud exists - it's real.
The government's and the Electoral Commission's solution is for voters to have photo ID. Opponents say that is discriminatory against 15% of the population who have no such ID when voter fraud is only at about 1%. Fraud at that level in seats where the majority is measured in the thousands is probably not such a big deal, but what about marginals?
I think that a half-way house method where a voter turns up with their voting card and something like a utility bill, council tax letter or bank statement for the same address would make a decent stop-gap method.
TBH I am more worried by postal and electronic votes than Dave turning up and pretending to be Bob then voting X instead of Y.