The UK always has a Leader of the Opposition ready to make the case that they could be the next Prime Minister. Technically we only vote for the representative

The UK always has a Leader of the Opposition ready to make the case that they could be the next Prime Minister. Technically we only vote for the representative in our local constituency, but who is going to end up PM is a big factor. We know what the options are before the election is announced: the current PM and the Leader of the Opposition (although in theory the leader of one of the smaller parties is also possible). Therefore no need for a primary process.