Gromit - "Many people in prison are there for arguably trivial crimes. Not paying council tax for example. It seems draconian to take those people's right to vote, for not paying a fine."
I am minded to agree.
The punishment for prisoners is just that - loss of liberty.
But for those that are going to be released into wider society, continuation of basic rights will hopefully assist in their re-joining society as useful and productive members of that society.
If you start building loss of voting rights into a punishment tariff, that can be the top of a slippery slope - what other basic rights should be denied on an ascending scale of criminal behaviour, and who is going to decide what liberties are removed for what crimes.
So I would argue, as others have done, that this is a matter of law, not common sense, and that is a better approach.