I live in North London, and since soon after 9/11 have watched twin-prop aircraft flying in large circles above the area at between 8,000 - 15,000 ft., above the airliners approaching Heathrow. At first they were RAF B/N Defenders, based at Northolt:
Since then I've seen the Caravan II, and on occasion what I think was a Piper Seneca or similar. The Defender and Caravan are both turboprops, but the Seneca is a turbocharged piston. They all sound quite different.
They're easy enough to hear if you're in a relatively quiet place and pay attention, but often quite hard to see, especially the camouflaged Defenders. All these aircraft were very active around the 2012 Olympics, but since then I've become gradually less aware of them. No doubt a good deal of ground infrastructure has been installed to take over this task since then.
Am I bothered? Yes, but actually I'm more bothered by weirdy beardies.