Obviously Police data isn't going to represent the whole picture, but nevertheless in the entirety of 2015 there were a total of 481 cases of alleged electoral fraud in one form or another, of which about 160 related to voting itself or to voter registration. Even most of *those* cases tended to lead to no prosecution either due to lack of evidence, or because the allegation was false, or because a caution is deemed sufficient. The remaining 300-odd cases are related to minor campaign offences such as including the name and address of the printing company on election pamphlets or making false statements.
So, in summary, there were around 150-odd alleged cases of voting fraud in one form or another across 2015. Even assuming this represents only around 1% or so of the total fraud, that bumps things up to around 10,000 possible cases. And that's probably an overestimate. For comparison, in 2015 there were over 50 million votes counted in all elections. The scale of electoral fraud just doesn't register.
This is not to say that electoral fraud is a problem that can, and should, be ignored -- but there is simply no evidence to justify changing policies to make the voting procedure harder for the millions and millions of people who neither abuse the system nor have any intention of doing so. Meanwhile, postal voting helps to widen access to those people who for one reason or another are not able to make it to their local polling station on the day, and there's no justification for ditching it either.