They don't always return the results in a timely manner. The free location near me can take up to three days to send the results. Far too late if they are "inconclusive" or otherwise not usable.

As an update to my OP. Verifly has updated it's UK data and seems to be managing trips like mine correctly. Added Vaccine card and it approved it and approved/waived testing requirement. Seems to know I'm a transit passenger as it is not asking for proof a 2nd day test has been purchased.