Where can I get a Covid PCR test and a fit to fly certificate near Cornwall or London?

Hi , need some help on where best to get the Covid pcr test. I live in cornwall so trying to find a clinic near me or in London as thinking of staying at Heathrow day before I fly. I have contacted a couple of clinics near me who have advised to do the swab myself and take it to the lab for a faster turnaround. I thought it had to be done by a health professional but being advised not ? also regarding the fit to fly certificate is this something that can be done separate to the Covid test ? anyone who has done this recently your advise will be much appreciated !