SND, So any clue to whether or not is this a requirement for routing into the LHR CTR to EGLW?
I note that the "abbreviated flight plan" will be discarded if not used. How long after the notified time, I wonder. I note that the accuracy should be +/- 15 minutes of the ETA - this is far too tight for a VFR flight plan where the actual departure time is very often subject to the vagaries of "actual passengers turning up time".
All very well for those with a manned ops department, not practical for those of us without.