That is high so close to coming out of LHR! Over the last weeks there seems to have been many cargo 747's visiting LHR and I've noticed that quite a few come in from far-away and then seem to hop over to somewhere near on the continent and thus being light seem to 'hammer it' on departure. Probably only saw them hit 12,000ft by your neck of the woods mr T, perhaps the captain of your 16,000ft 747 is an ex-U2 driver ? ;-)