Argument: There is no state interest in denying prisoners a vote
Daniel Machover. "Prisoners too should voteToday's agenda". The Guardian. May 11, 2001 - Any supporter of imprisonment who was asked to explain the purpose of sending some criminals to prison is hardly likely to say "to stop them from voting while they are out of circulation". On the other hand, banning someone convicted of election fraud from voting, whether imprisoned or not, does make sense as a punishment.
So parliament could in fact give sentencing judges discretion to ban voting on a case by case basis - several countries do this already. That's a far cry from the blanket "punishment" of a voting ban on all sentenced prisoners.