Until major London airports have sufficient capacity for it to be worthwhile letting in small planes it's logical for London to be served via a smaller airport with less capacity issues, and for the hub connection to be where all the other small UK regional airports avail themselves of the current alternative to Heathrow's 3rd runway; Schipol.
Of course all this means that both services are made less viable, and that border staff get involved; a perfect example of the harm being caused by political dithering :-(