> To a Brit, the idea that an outgoing US president remains in office for weeks even if their defeat is confirmed

It’s one part premature calling, one part Electoral College and one part Congress’ timeline.

Premature Calling:

The United States is huge and we don’t have one Federal Election, we have fifty-one which are relevant to the Presidency/Vice Presidency and fifty which are relevant to the voting members of the House and Senate. There are also additional elections for non-voting members of the House from DC and the five territories.

Some of those elections are not certified on Election Day, not because you can’t tell who basically won, but as a matter of process you really do have to count every vote and each jurisdiction handles the logistics of that not just for the Federal Elections but the States’ own elections.

Electoral College:

We don’t elect the President. We elect the mooks who will then go on to elect the President and Vice President through the Electoral College. Those mooks are appointed by Congress depending upon the outcome, which is a winner-takes all popular vote in 48 States and split in 2 States where the winner of House seats in two States secure their party’s nominee one electoral college vote each. The Electoral College vote quantity that comes from those States’ Senate apportionment (2 per State) are still winner take all, and I believe both of those States still only have a single House seat each (EC vote quantity is House apportionment + Senate apportionment) so in practice it affects 2 total Electoral College votes across all the States, but I’m going off memory so someone can correct me if I’m wrong.

States must have their slate of electors appointed 6 days before the Electors are due to meet in their respective States, and this year they are meeting December 17th. When they meet on that date, they’ll vote for both President and Vice President.

Congressional Timing:

The States will then send the votes to Congress and they will be received by the Archivist by no later than December 25th. The next Congress will resume business on January 3rd 2025, at which point the Archivist will deliver the results of the Electoral College votes to the new Congress who will deliberate on January 6th to count and certify the votes.

Once that’s done and if Congress doesn’t have to decide a tie, we officially know who the next President will be, and 14 days later on January 20th he will be sworn in at noon.

It’s slow, tedious, and given faithless electors are usually prosecuted in their States, if not having their votes declared invalid, we could probably save a lot of time by removing them from the equation, but some reforms would require a constitutional amendment and others wouldn’t, so inertia rules the day.

The Westminster system has some advantages, but you also have a Head of State divorced from the scene of electoral politics until the very end when the next Prime Minister—a Prime Minister-elect at this point?—asks the King permission to form the next government. We do not, so this is the compromise we’ve developed and somewhat refined.

https://www.archives.gov/electoral-college/key-dates

This has the full timeline from Election Day to Inauguration Day. Not on there is that mail-in ballots for States that have them arrive sometime in the preceding month, but thats determined by their own laws.