There's a similar situation up north.
Glasgow (a BAA airport too) has been running a similar scheme since last year but it's not run by LAAS or any other recognised spotter organisation.
It is more like a local co-operative between the poilce and the spotters with BAA made aware of the activity.
I can't tell you how successful it is or what the local spotter/shooters think about it. Maybe Fred Seggie (FredS) will see this and add his thoughts on the current set up.
I'm not involved in any scheme at my local (Edinburgh, also BAA). I haven't been approached to join nor am I aware of any scheme.
It's just the 'Report suspicious behaviour' notices here and there.
Both our airports have one major problem, just like LHR
The denial of proper spotter/photography areas, so the result is we are scattered all over the perimeter.
That is the fault of the BAA, making the Police work much more difficult, for sure.