The minimum mandatory day 2 antigen test required for returnees to UK is available on many sites but the best (lowest) value deal we found is on expert-medical.co.uk at £28 pp.
You are immediately emailed the code required for your Passenger Locator Form and the test pack is waiting for you on return.
Chris
Ps
Interestingly the antigen test is recommended if you’ve had COVID as the full pcr test can show positives for up to 90 days