It's worth pointing out that citizens of other EU countries can register to vote in the council elections, even though they don't have a vote for the local MP.
The council has a hotline to help with registration. The deadline is Tuesday 20 April. From the council website:
People can phone 020 8359 4111 Monday-Friday, 9am until 5pm, and then from April 26, until 8pm, for assistance with voting issues.