Not necessarily. Either: two-thirds of the House of Commons votes in favour of an election, or one-half of the House of Commons votes for a Vote of No Confidence and no other government is successfully able to form, or one-half of the Commons followed by one-half of the Lords votes to repeal the Fixed Term Parliament Act.
First is most likely, seeing as Labour have said they'd support an early election.
If the HoC voted for an election today it could happen on the same day as the local elections if I am counting correctly..? Not sure if it'd be possible to organise things so quickly though.