this came up last year yes and no; PCR and RT-PCR tests are actually different tests. the RT stands for "Reverse Transcription".
"A reverse transcriptase (RT) is an enzyme used to generate complementary DNA (cDNA) from an RNA template"
it is only possible to detect the SARS-CoV-2 virus using the RT method. therefore the "PCR" test performed by the London clinic will in fact be an RT-PCR test. but for some reason the clinics in London have decided to use shorthand and have dropped the "RT" just to confuse everybody. I guess they year for days of yore when they were a world power ????
last year there was at least one report of a traveller being held up at check-in, and rightly so, because the certificate said "PCR" but the IATA rules said "RT-PCR". check-in and immigration staff shouldn't be expected to be scientists.
however there have been no recent reports on here of travellers to Thailand encountering this issue.