I'm working and living in the UK, what date should I enter for this question when completing the Physical Presence Test "Enter the date your 12 month period ended"?

If you're using the "bona fide resident" test there is an option to indicate that residence "continues", but when using the simpler physical presence test you just need to provide a date that encompasses a full 12-month period for the form to compute and be completed correctly. 

For many folks this is simply 01/01/2024 to 12/31/2024 (present the entire calendar year) but for folks just starting (or ending) residence this may instead look like 08/01/2023-08/21/2024 or 03/01/2024-03/01/2025.