Re: what are they doing?
"Are they using these to spy on Brits? "
We, like the rest of the "civilised" world have domestic agencies for that. Separation of army and police, all that jazz.
The Met has taken a leaf out of certain TLAs operations manual, and runs it's spy planes through front corporations with PO Box contact details to avoid having to discuss exactly how much of this targeted spying whatnot goes on.
Luckily there are some seriously beardy plane spotters out there, and a light plane with a camera dome in it's belly is somewhat noticeable. The people who make the camera;s also aren't shy about tooting the horn over their products capabilities, such as ANPR.
It's a technique that is certainly used in the US, and almost certainly by the other five eyes. Exactly how legal certain aspects of it are (stingray) are up for debate, but cops with a camera in the sky are generally accepted as being legal.
"why is the army flying surveillance drones over civilian areas?"
Easements, or it's crown land. I get a form letter from the MoD about my Welsh "farm" where they notify me when they are planning on sending some squaddies through, with who to contact if they fuck something up. Or "accidentally" kill a sheep, and then cook it up, as soldier have done since time immemorial.
They also have a standing notice that they might fly planes over, which is why it's not much good for anything other than subsidy collection. Well, any locals who want to keep their sheep on it can, in exchange for a spot of mutton once a year.