Peter,
I hadn't thought about it until you posted, but it would certainly seem reasonable to file a flight plan from a ZZZZ airport (i.e. a random lat/lon), depart Shoreham VFR and then activate the flightplan when you are near that lat/lon.
Returning is probably more complex in that HMRC et al use the flightplan to understand where you are going to arrive. Once in the Olympic period, I would try a Z plan with the transition prior to the UK FIR. This certainly achieves the spirit of the regs (not over burdening London Control with too many aircraft and not having lots of people cancel IFR one waypoint before an airport and then stooge around at low level.
I suspect as well the IFR system for the small/distant airports will loosen up a bit (or may have very few slots booked).