https://www.heathrow.com/at-the-airport/fly-safe/covid-19-test mentions companies that do COVID-19 testing at Heathrow but I'm not seeing those providers at https://www.gov.uk/find-travel-test-provider . Does the UK government accept the test results provided by the testing providers at Heathrow?
I have used ExpressTest (one of the companies listed on Heathrow's website) several times, for arrival and departure from the UK, and it worked fine for me each time. They sent the number to put on my passenger locator form by email shortly after booking, and it was accepted by the government website. (For completeness, I have used their Heathrow drive through site for departure tests, but I have only used a different ExpressTest site for my day 2 arrivals test.)