I can only assume we are dealing with positioner flights within the London TMA here?? We deal with many Biz Jets, say going from Luton to Heathrow during the afternoon rush. Given that the delay into Heathrow at this time may be 10>15mins, it is not un-reasonable to delay this aircraft "on the ground", and then having taken the delay on the ground, get airbourne from Luton (in this case) and go "straight in" to Heathrow. Thus the Supervisor may give an "airborne not before time" to achieve this objective.