When the ECHR said the UK were breaching prisoners human rights by not allowing them to vote, Parliament discussed it, voted on it and subsequently ignored it.
It always helps in an argument about the ECtHR to actually cite what they said, rather than what the Daily Mail told you they said.
The objection raised by the ECtHR over prisoner voting was not that many/most/all prisoners aren't allowed to vote. They aren't, to some greater or lesser extent, in most jurisdictions, and that's entirely fine. The objection was that in the UK, there has never been debate to determine who can and can't vote: you just can't. The court held in Hurst that that the UK government should make it clear what the rules are, and pass legislation accordingly which is compliant with Article 1: essentially, it's OK to deny people the vote providing it's been discussed and voted on. That happened, and the issue is now settled. The basic contention is that if you're going to deny people what would in general terms be their rights, there has to be some sort of due process.
People who bang on about prisoner voting making them physically sick, as Cameron claimed, should take heart from Scoppola (No. 3) v. Italy which held that a lifetime ban on voting for particular classes of offences (something far harsher than UK law, where murderers on life license can vote), was entirely compliant. The government was just grandstanding over Hurst (pandering to the sort of UKIP illiterates who think that the ECtHR is part of the EU, or that Britain's membership of the ECHR and the Council of Europe arises from our membership of the EU) when it could trivially easily have passed legislation along the lines of Italy's and had done with the whole issue.
Facts of the situation: www.echr.coe.int/Documents/FS_Prisoners_vote_ENG.pdf