It would depend on the definition of transit though? If an individual transit was entirely airside and they did not pass the UK border then there would be no testing, either prior to departure from their point of origin, prior to arrival in the UK or prior to their departure, just the PLF.
Given the state of airline schedules it is entirely possible that they had a connection that was overnight, or required going from one airport/train station to another or an individual might be "self-connecting" on separate tickets. These cannot be completed airside and would require the PLF and a pre-booked Day 0-2 test. They are still considered transits for the purposes of the PLF though they might not meed the conventionally accepted airline usage of "transit", "connection" or similar.