TC, you're having a laugh, aren't you? Debden doesn't see very well down that way and we all know how well the Stansted radar doesn't see Cirrus (especially those which are working Thames Control) in that area.
As for 'Northolt isn't piped into the NATS Network', I would hazard a guess that Farnborough has only recently gained the luxury of more than one or two radar sources so there has probably been a fair bit of piping-in done already. There is only one primary radar that has good solid coverage there; I suspect the reason behind radar selection is more business based than common-sense based.