I was being a bit over the top really! Anyone can use Heathrow subject to having the required landing/take off slots. There is a criteria for the allocation of Ad-hoc slots and GA comes quite low down the list (still).
The only slight change is that post-11/9 the number of scheduled flights has decreased freeing up more slots for the ad-hoc pool, and as a result GA.
I'm sure come the summer it'll start to become less available - depends on the airlines.