I want to fly into the UK for 1 day (from the US, also I'm a UK citizen), and then leave for another country. Do I need to pay for the test which they require me to do before or on the second day and do I need to stay in the UK for 10 days?
EDIT: It says that I still need to book the tests, but will I be fined and imprisoned if I don't take them since I will be in another country?
Note that those are the rules for England. They are different in the other countries of the UK.
Note that they can fine you directly at the border when you arrive if you haven’t booked and paid for your tests. I suppose you could also be prevented from boarding your first flight, though I don’t know if airlines check that.
The answer from jcaron is wrong. I flew to UK and left in 1 day. The border police told me I just had to write that i'm exempt and show the plane tickets for the next flight to prove I am indeed going to leave. You don't have to book a test if you're in the UK for 1 day.