I just got stuck in LHR (meant to be in transit) overnight as the flight to the US got cancelled. I stayed at Hilton and got a Halo test. They had results by 4 am (must be in by 6 pm hotel said..) I had a previous test done but was out of the 24 hr period... They checked the time/date on check-in. I don't think my then 48hr test would have worked.