The polls close at 23.00. UK and Scottish Parliamentary Elections, we will know over night. The first results come in by 23.30. I have seen it being confidently called as early as 0300 with the Tony Blair landslide.
The last results are NI because they do not count over night and wait till the next morning. Even with a recount, it is rare not to know a result by mid day the next day.
To elaborate a little on the UK elections method.
All eligible voters must register at their home address and have their names put on the electoral roll. If your name isn't down you don't get to vote and registration must be done well in advance.
All registered voters are sent a polling card which speeds up the process at the polling station but isn't necessary. Each registered voter then goes to the polling station and is given a ballot paper. You write an X in the box of your preferred candidate and fold and put it into the ballot box.
There are LOTS of polling stations manned by volunteers and there is rarely any wait time to cast a vote. they are open 7am till 10pm on election day.
There is also a postal voting method where you can mail a ballot in if you are unable to vote in person on the day. You apply several weeks in advance for a postal ballot for any reason.
These are mailed out to voters 3 weeks before the election. You have until 10pm on polling day to return it via the post. If it's later than that it does not get counted.
Returned postal ballots and on the day ballot boxes are kept sealed until after the polls have closed and are taken to counting locations. Usually by local councils.
Once the polls are closed the boxes are opened and counting begins. There are registered observers from each party that are able to observe the counting process to make sure it is all above board.
How long the counting takes depends on how many votes have been cast, and how many people there are counting.
All of the votes are counted by hand.
In the last election in 2019 a total of ~32million votes were cast.
First results are usually known an hour or so after polls close, all of the counts are usually complete by 6amish the following morning.
UK parliament has 650 seats in the House of Commons. Each constituency in the country votes for an MP in one seat. Whichever party gets a majority forms a government and wins the election. Whoever the party leader is becomes Prime Minister. It's a FPTP system (sadly) so as soon as one party gets the necessary number of seats the election is called by the TV networks.
We don't have "decision desks" a seat is only decided one way or another once all the votes have been counted and verified.
The count in the UK is much quicker than the US because we use simple paper ballots. You just put an X or a n other mark in the box of your preferred choice and put it in the box. If you make any identifying mark, or mark more than 1 X your vote is invalid and not counted.
There are no voting machines or counting machines. All the votes are counted by hand.
If a count is very close and a recount is required then all the votes are counted by hand again. A result in that seat will not be declared until the recount is complete. Recounts do take longer and in some cases a result will not be declared until lunchtimeish the following day, but I don't ever remember it taking longer than that.
