I only saw them departing from 10L (now 09L); I was working at the radar unit on the north side ('London ATCC') during most of 1970 and the car park (or the canteen if I'd arrived early) gave an excellent view. They would roll from just outside this radar unit (which was just west of the tunnel) heading east; line up, full throttle and go; no slowly opening up the engines and as Pax says, slowly easing itself off the ground.