Perhaps the greatest irony about prisoners voting rights is currently there is a case from Austria having just gone through Europe called the Frodl case.
This actually allows a voting ban to be placed on those guilty of electoral fraud or abuse of public position, so any MP who gets sentenced to prison for lets say false expenses could automatically lose their rights to vote while convicted rapists and murderers are allowed to vote.
http://www.telegraph.co.uk/news/ukne...an-ruling.html