How do you oust a British prime minister?

Answer 1

Normally, they leave office if their party is voted out in a General Election, although they can resign the position for a variety of reasons, such as ill-health (as happened with Harold MacMillan and Harold Wilson) or if they have become too unpopular with the public for it to remain tenable for them to stay on (as was the case with Tony Blair and Margaret Thatcher).

Unless they have themselves lost their constituency seat in the election, they have a choice as to whether to remain in politics as a member of the opposition (and maybe as a member of the Shadow Cabinet), or to move on to another job in the political arena such as heading a policy research committee, etc. Some Prime Ministers are ennobled after their tenure is over, and move on to sit in the House of Lords. This can also happen to leaders of the opposition, such as former Labour leader Neil Kinnock.

Answer 2

Usually they are voted out of office. An election must be held every 5 years, but the Prime Minister can call one earlier. Sometimes a Prime Minister may retire when he is in office, or he might die in office.