That revised clearance is only given to CPT departures. The CPT departure route actually gets too close to the final approach to 09L, so instead we give a heading (usually 220) to those on a CPT route. We don't give it at start up because the heading is chosen by the radar controller tactically. If it's busy with aircraft flying downwind to the south of Heathrow, it might be 220, but if it's quiet, radar might give a heading of, say, 260.
DVR, DET, MID, SAM routeings are not affected by this, and follow the departure route as given on start up. This only changes when there is a helicopter operating to the south of Heathrow, then all southbound a/c (CPT, SAM, MID, DVR, DET) will get 'straight ahead to altitude 1,500 feet before turning right'