It needs a two thirds majority to call an election, it's no longer in the gift of the PM. No way would Labour agree whilst settling in a new leader.
Didn't that get changed recently? The 5 year fixed term is no longer a thing.
Edit: yep, the Fixed Term Parliaments Act was repealed. We are back to the old way: the PM can, at any time, request a GE of the Queen. In practice, the Queen is never going to refuse so the choice of when a GE takes place is back in the hands of the PM.