In common parlance, rapid tests are antigen tests are RATs. PCRs everyone understands but there is a processing time and overkill for the UK requirements. HMG doesn't hugely care, similar to the USA, it just wants some evidence that a half reliable test was taken just before travel. The Statutory Instrument has various specifications attached to it, but so long as you get a piece of paper saying "negative" then that's good enough. See the main UK arrival thread in the Coronavirus forum for dozens of data points, the question keeps coming up there too, but the essence is get that certificate, ideally in English, and all will be well. It's not a big deal. LFT, NAAT, LAMP, PCR, they are all OK and most larger drugstores can do them on spec. The reason for being relatively laid back about that is because the focus is on the day2 test, which HMG does regulate more strictly.