Enshrined in the European Convention of Human Rights is the right to go where you want, live where you want, etc., but not while incarcerated. While serving a sentence in accordance with a duly legal court of law, one should have to obey the strictures of that sentence. In the UK and the US, felons lose their rights to go as they please AND vote.
Don’t do the crime, if voting is such an important issue to you! I cherish my freedom, for example, so I don’t commit acts that would have me jailed and not free anymore. Simple as that!
Now, according to Napoleonic Law (France), one is guilty until proven innocent. Seems far more against the “European Convention of Human Rights” than not being able to vote in prison.
The Schengen Agreement is not officially part of this.
The parts of that convention referring to such things were signed by the UK but not ratified.