Yeah, but in the UK, you don't have to (and can't) register your affiliation. That's a matter between you and the ballot box.

There doesn't seem to be a concept in the UK of "registered voter". You're either on the electoral roll or you're not. It's the same thing, but there's (AFAIK) no term for it.

The electoral roll is also used by credit companies and insurance companies, to check that you live where you claim (or something). There's a box on the registration form that you can tick (check) to prevent wider access to your information.

AFAICT, you can be registered on more than one roll, but you're still only allowed to vote in a single ward.

Personally, I'm on the electoral roll, but I've never voted. Until we get a compelling opposition party, I'm unlikely to.