I think there has been some confusion over this issue recently.
It has always been the case that it was compulsory to register to vote, and the maximum penalty for failing to do so was Β£1000, although you would need prosecuted and fined by the courts.
In the past however, each household was sent a form, and it was the responsibility of one person to fill it out with all the details of everyone else in that household eligible to vote. If you lived with your parents then, you may well find that they did it for you in the past. If you didn't fill it out, after getting a few reminders, you would find a 'canvasser' arrive at your doorstep to do it for you there and then. It was only those persistently evading the requirement that would be prosecuted. You would also find it very difficult, if not impossible, to obtain any credit if you weren't on (at least the edited) roll.
They changed the system recently so each individual is now responsible for there own registration; very few people were prosecuted under the old system as it was difficult to identify who was at fault - remember, it was norm that often someone would register you on your behalf, and people would always argue they had been told someone at the address had registered them.
With the switch to IER, the government also changed it so that councils can issue a Β£80 civil penalty, like a parking ticket, to those failing to register, to make it easier to enforce.
TL;DR: Yes, it is compulsory, but it always has been.