I have seen a blacked out van driving around the local streets. The side door is open and there is a man in there with a camera type device pointing it at the houses as they drive past.
Take its number plate and report it to the police..... although it might be filming for Google Street View, they ought to tell people if they're doing it.
Why? It's perfectly lawful to film people and places while you're in a public place (such as on the highway). Even if the cameras are taking pictures of people in the street, in their gardens and in their homes it's entirely legal (and long may it remain so).
Anyway, it's most likely to be part of Royal Mail's current countrywide mapping programme.