Covid-19 restrictions change at short notice, but often with a few days between their being announced and their coming into effect. The applicable rules that are in force at the time you enter the UK continue to apply.
E.g. announcement (of easing/tightening of rules) on 27 July with effect from 3 August.
- Rule changes (unless otherwise indicated) are from 04:00 (GMT/BST depending on time of year) of the day from which they are effective.
- If you arrive in the UK on 2 August, the previously applicable rules continue to apply regarding quarantine/test to release etc.
- If you arrive any time after 04:00 BST on 3 August the new rules apply regarding quarantine/test to release etc.
H/T: British Embassy, Vienna for clarification.