I've used Global Health Travel clinic, it depends on which method they use as to when you'll get your results. The swap up the nose and around the back of throat = 12-24 hours. The saliva test they have to send off to a lab = in the early hours of the next day. Both PCR tests.
I've also used the
Fleet Street Clinic in London, F**k Off expensive (£495) but if you're willing to pay you'll get your results same day, as in 2 to 6 hours later.