Not being on the electoral roll may be a civil offence, but it is not a criminal offence.
Councils send out an annual canvass to every address, where someone in control of that address must respond giving the names of people who are entitled to register. It is a criminal offence not to respond to this truthfully. However, if the time for the next canvass has not yet come and you have not received one, no offence is committed by not volunteering the information.
When the annual canvass is returned to the council, it will identify whether any individuals named, who are entitled to register, are not registered. Under the Electoral Registration and Administration Act 2013 Section 5, the Electoral Registration Officer must then invite those people to register, and the ERO may impose a deadline by when they must register, and if they fail to register by the given deadline then the ERO may impose a civil penalty.
If someone responds untruthfully to the annual canvass, and as a result a different individual who is entitled to register is not known to the Electoral Registration Officer and therefore not invited to register - then an offence is committed by the person who provided the false information, but no offence is committed by the person who has not registered.
You can of course register to vote voluntarily without being invited to register, and if you provide false information that is a criminal offence.
The electoral rolls are held by each council. The full electoral roll is available to financial institutions such as banks and insurers, and other organisations including political parties, government departments and TOCs that want to confirm the existence of names and addresses provided by possible fare evaders. I believe it is possible for anyone to view the full electoral roll in person at council offices, but am not sure.
The edited roll which is opt-in is available to commercial organisations for advertising and research purposes.