The court has ruled that Italy can legally deprive prisoners of the right to vote because it's not a blanket ban but that the British government's ban on voting is illegal because there are no exceptions. It has given the British government 6 months to change the law to allow prisoners to vote.
The policy director of Liberty, Isabella Sankey, has criticised the ban on prisoners voting and seems to suggest that because it is an old law (an "irrational Victorian law) then it must automatically be wrong. Liberty, of course, campaigns for the protection of the fundamental human rights English people have possessed since the writ of Habeas Corpus was first used in the 1100's. Are they suggesting that Habeas Corpus, Magna Carta and the Bill of Rights should be be repealed because they're old?
If you commit a serious crime you will (or more accurately these days, may) be punished with the temporary suspension of certain rights that society has, over the course of centuries, decided should only be enjoyed by those who respect the law. These rights include the right to liberty and the right to vote. If you can't live in a way that the rest of society finds acceptable then why should you have a say in who makes the laws to (theoretically at least) ensure the population behaves in a way that society deems acceptable?
Membership of the EU requires submission to the primacy of the EU Court of Human Rights and the EU Court of Justice. Technically they aren't organs of the EU state but in all practical terms they are merely arms of the EU and EC, there to ensure that EU laws are adhered to by member states.