We’ve booked DAM Health at 37 St John’s Hill for our 24 hour PCR test before we travel at the end of this month. It was straightforward and under 17 is free with parent booking. Not cheap but probably the standard rate. You can book online.
We used London medical laboratory when we went to Portugal.
Even though it was bank holiday Monday, we got our results within a few hours and they followed up v quickly on my frantic emails. Not cheap and not local but would use again