Latest update 10/02/2022:

From 4am on 11 February all testing requirements will be removed for eligible fully vaccinated arrivals, with only a Passenger Locator Form (PLF) now required.

If you are not fully vaccinated:

Arrivals who are not recognised as fully vaccinated will only need to take a pre-departure test and a PCR test on or before day 2 after they arrive in the UK, as well as filling out the PLF. They will not need to self-isolate on arrival, and will only need to do so on receipt of a positive result.

Find out more by visiting the gov.uk site.