Could it not be a case that you can vote from prison, depending on the crime you are serving time for?
Scenario 1:
A regular bod like our good selves finds themselves in a situation where you are facing prison for defending your own property. You have smacked the burglar in the face but he's gone down like a sack of shit, give himself a bit of brain damage and you've been done for GBH with a 3 month sentence. During this sentence its voting day. You should get the right to vote.
Scenario 2:
Old Doris has refused to pay her Council Tax for the last 5 years running. This year enough is enough and the magistrate has given her a 2 week sentence. During those 2 weeks its voting day. Doris should get the right to vote.
Scenario 3: Ian Huntley takes the lives of 2 innocent, beautiful little girls. As he does that he takes away their human rights, thus removing their right to vote in the process. Ian Huntely should not get the right to vote.
See what I mean?