It's a bit of a shame that Collinson is not at T5 anymore. It was rather good organized and my results always came after about 12 hours.
Only once a PCR test failed but received a proactive call 12 hours after test and they've booked me another test which I could do 2 hours later at no cost.
Given that I think I'll stick with them for my pre-departure test and just go to LCY or St Pancras (coming from Richmond traveling to LCY takes as long as going to T5 if using public transport, believe it or not
)