As of 4:00 am on the 7 December 2021 the UK government placed mandatory procedures for those entering the UK from a foreign country.
All passengers entering the UK must take a pre-departure test prior to travel. This must be done in the country you are travelling from (i.e., the start of your journey back to the UK).
Airlines will be required to check for pre-departure tests alongside a completed passenger locator form, and passengers will not be allowed to board a flight without providing evidence of a negative test result.
The full rules for travelling to England can be found via Gov.uk. Guidance for Scotland, Ireland and Wales is also listed at the top of this page. The information here also gives you links to obtaining a Passenger Locator Form.
When arriving in England or Scotland there is a cultural exemption, which can be applied for under certain conditions. This exemption is managed by Arts Council England.
This exemption applies to England and Scotland. Please check the relevant government guidance for Wales and Northern Ireland.
This exemption is not available for those entering England or Scotland from red list countries.
Whether you are eligible for the exemption of not you still have to follow the pre-departure protocols.