If you are flying outside controlled airspace and at a height which is not "known" to ATC no vortex wake separation will be provided.
If you fly at 2500 ft just outside the zone boundary you should be over 1000 ft below any Heathrow inbounds so you should not experience any problems. If you do experience wake turbulence you are encouraged to report the fact, together with any details you can provide of the aircraft generating it.