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.
https://www.legislation.gov.uk/ukpga/2013/6/section/5/enacte...
Maintenance of registers: invitations to register in Great Britain
(1)A registration officer in Great Britain must give a person an invitation to apply for registration in a register maintained by the officer if—
(a)the officer is aware of the person’s name and address,
(b)the person is not registered in the register, and
(c)the officer has reason to believe that the person may be entitled to be registered in the register.