TeeS, thank you for the info above.
I subsequently called BorderForce and was informed to just do both my Flight Plan and GAR between the points of the private site in Kent and Le Touquet (LFAT). All is straight forward, so long as all POB have UK passports. Should any passenger hold a non-UK passport, then BorderForce must personally arrive and meet you upon landing back in the UK.
I’ve flown back in to the UK quite a few times with non-UK EU engineers on board (listed as pax in the GAR) and have never been met by anyone.