On December 12, 2019, UK will hold a general election. As stated in Wikipedia's 2019 United Kingdom general election:
The election is to be contested under the same boundaries for 650 constituencies that have been used since the 2010 general election
This means that the 650 members of the Parliament are elected individually, each one in a different constituency.
Some polls show that the current primer minister, Boris Johnson, leader of the Conservatives, does not have his seat granted in Uxbridge and South Ruislip, the constituency he is running for:
Wikipedia on Prime Minister of the United Kingdom states:
The Office of the Prime Minister is not established by any statute or constitutional document but exists only by long-established convention, whereby the reigning Monarch appoints as Prime Minister the person most likely to command the confidence of the House of Commons; this individual is typically the leader of the political party or coalition of parties that holds the largest number of seats in that chamber.
However, I cannot find a reference on whether the Prime Minister needs to have a seat in the Parliament. Is there any rule on that?