This is a slight thread creep; I admit. London airports have a TA 6000'. Descending into LTN from the north I have often been cleared descend FL60. I queried this with ATC, but they didn't have time for a lengthy explanation. The best I could get was "the lowest level available on the airway was FL60. At this point we were outside the London TMA and thus above the FIR where the TA was 3000'." It it still the case. I trusted NATS, but you'll understand my confusion as I was descending towards the London TMA. Opinions?