At the moment ExpressTest Fit to Fly are not ok for Day2 tests. See the main thread for this topic, UK Arrivals, in the Coronavirus and Travel Forum. Lots of discussions and datapoints there, but to summarise, that test won't generate the reference needed for the PLF. There are other tests available, including next door in the Sofitel, which are much quicker. This area will probably get better in the next few weeks.