I struggle to see the logic of intentionally not using the services of a nearby ATC unit
Well most folk probably would if there wasn't any hassles. But the unit has dug themselves a hole where by the GA traffic can't see the point of talking to them. If your likely to get no service or a rude unhelpful one your not going to speak to them. And you can scream airmanship all you like a couple of bad experiences with a unit will put off most pilots talking to a unit if they don't have to. Maybe the reason why traffic is wanting to stay with Farnborough. It should be good airmanship for the unit to promote good relationships with all the pain in the arse traffic.
And to be honest its not really the problem of the pilot if the unit is too tight to provide a secondary radar feed.
And how the hell is a pilot meant to know what type of sodding radar the unit he is talking to is using? especially a VFR spam can or plastic pig. VFR pilots won't have a clue about the decent profiles on various approaches or for that matter what it actually means. For all I know the 600ft AGL at 10 miles could still be inside the theoretical "danger area" of a none precision approach. Again the pilot wouldn't know which procedures are being used unless they speak to the unit.
This all sounds quite unsafe to me especially if there are lots of microlights kicking around with dodgy primary returns.
I had in the past a similar problem with a Mil radar unit and more by luck than judgement I was chatting to a SRG controller inspector type person who had thought that the GA service in the area was fine and working well (as he had been told by that unit). When he found out what was really going on it was sorted pretty quick. Neither side got what they really wanted but it was a hellava lot safer for all concerned afterwards.
I would get a group of you and contact the SRG and arrange a meeting. You will have to give something's away maybe standard joining routes etc but if its as bad as you say you can't let it continue until god forbid there is a fatality or fatality's (you choose the number, whats the largest CAT going in there?).
The unit manager will be fuming anyway as he will be reading this, so if there was bad feeling before it sure as hell won't be any better after publicly broadcasting his units dirty washing.