Agree Not Long Now, and it has been looked at several times. As with these things, there already are SIDs that finish with a FL (cannot remember where, EGNX??) however so far the CAA have rejected the idea for the London TMA. I cannot remember all the reasons why but will attempt to reraise the question and find out. One reason that was certainly raised was when the aircraft will switch to 1013, obviously the differences between UK and other places already exist so a safety case has enough clout with just that on its own.