[lawtalkinghuman]

It isn't a criminal offence. It's a civil penalty that can be levied if they require you to register and you don't.

The legislative text (it's an amending regulation—think a diff—but the underlying statute hasn't been updated on Legislation.gov.uk yet) is here: https://www.legislation.gov.uk/en/uksi/2013/3198/regulation/...

The Electoral Commission's guidance - https://www.electoralcommission.org.uk/running-electoral-reg...

I can't find any reports of people actually receiving the penalty. It'd certainly be useful if the Electoral Commission collected these statistics, but they don't. It seems fairly rare, and it's likely to be one of those things that's there to encourage people to register to vote rather than something to be actively enforced.

I'd also note that given there is a requirement for an in-person home visit to encourage registration before service a formal notice, it's pretty likely that COVID restrictions and guidance meant quite a lot of the home visits didn't happen over the last few years.