Calling an election is now back in the hands of the government (not parliament) since the repeal of the Fixed Term Parliaments Act; a PM can ask the monarch to call an election without having to get the agreement of parliament. Neither the current nor next PM (whoever they end up being) is likely to do this if the polling numbers are anything to go by, of course. I think the only other way an election can be called is if the government loses a vote of no confidence.