I've spoken to a couple of people recently who are not on the electoral register (due to recent moves) and have told me that it's too late for them to register to vote. Nothing could be further from the truth. In fact if you're not on the electoral roll you have until April 20th to ensure you can vote in the coming election.
April 20th is also the deadline for registering for a postal vote, so if you're going to be away or very busy on the day of the election you can still vote by contacting the council's elections department and registering for a postal vote. You don't need to give a reason you just need to apply.
To register to vote call 020 8314 6086 or check out the council website for details.