I used this mob on recommendation of a friend:
https://www.privatecoronavirustests.com/

They have a walk in clinic for tests in Putney and Guildford (I used the Putney clinic).

UKAS accredited lab, I took the test last night at 17:40 and had a certificate (negative) emailed back to me today at 13:32.

They say 97-99% of results are back the next day after they’re received, and you pay an extra £30 (£179 total) to have the test taken in clinic rather than posted in. They say the 1-3% is for false positives (or I guess any positives?) which get re-tested to account for false positives in testing

Hard to guarantee anything in these days but was pleased with the service and the turnaround. Obvs 1 test is easier than multiple etc etc.