No problem.
Surely all they need to do is to comply with AIC049/2011. If they plan to remain outside the "inner area" of the LHR CTR then they can call "Special" on the ground.
Otherwise, if operating anywhere from about NNW round to the ESE of Ascot they need to phone LTCSWA to obtain takeoff permission, giving the lat/long and exact departure time. This can take a mere twenty minutes or so and may result in an "equivalent delay" to that applied to inbound IFR traffic to LHR.
Should make for a real action packed weekend. The locals will obviously love it, especially those relaxing in their quiet country retreats with Olympic sized swimming pools....
