I've Seen several light aircaft around the West London Area, Near RAF Northolt and London's Heathrow. I once thought these were road traffic monitors as when their was congestion on major roads suchs A40 or the M4 they would circle around, but I havent seen any for a while.
I have heard that Heathrow gets a few light aircaft really early in the morning between 04:30 and 06:00.
I have seen the Grey RAF Islander doing circuits around Northolt which my friend has noted of PPRUNE before.