Customs etc
Quote from IO540
"OK, non-customs airports are much cheaper, but that is not an option flying in/out of the UK."
I think that it is, as you are probable well aware !
You do not have to leave or arrive at a full customs airfield in the UK. Popham and a hundred other airfields are quite acceptable airfields to depart from or arrive at from a foreign flight.
Vote with your wings and avoid all of these overpriced airfields which suffer from delusions of adequacy !