I don't see why this can't go ahead with minimum disruption to the existing prison regime. Ballot papers would be smuggled to the cons hidden in prison officers' bottoms, and would be exchanged for sexual adventure with the con's missus; completed ballots would be smuggled out by bent solicitors hidden amongst case papers. Wrapped in strong plastic, natch. Candidates could throw bundles of election addresses over the wall into the exercise yard, or use the capacious bottoms of bent screws to smuggle in PAYG mobile phones with ringtones fixed to the 'Red Flag' or something by Elgar. Packets of heroin dyed either Tory Blue, Labour Red or LibDem Yellow could be carried in the cheeks of visiting girlfriends and exchanged during the parting kiss. All in all, no change there to our existing prison regime.
It'd take a while to complete the exit polls, though.