[mrweasel]

That won't actually make much of a difference in countries like the UK. The prime minister is not an elected role. In the UK there are so few parties that you'd have to either not vote or vote against your conviction to keep a party's prime minister candidate.

Technically you don't even need to be on the ballot in some countries to become the prime minister. Granted it would be weird, but ministers are appointed by the party or parties in power in parliament, so there's no rules that says that any minister needs to be elected or even on the ballots. It happens not to infrequently in parliamentary countries that ministers are pulled in from outside. Normally they'll the run in the next election, otherwise they'll have no voting power in the parliament.

I can see the argument that it's not democratic, but it avoids the issues of a presidential election as we seen in the US. In the end, the prime minister is "elected" by the candidates the people voted into the parliament, it works best if you have more than the four parties you seen in the UK.