So, what you are looking for is a site (not a sight) which is open enough to be a sniper's paradise. It seems that you want to stack the odds in your favour.
You might enjoy Splatoon, which is near Wickford in Essex, although it might be too far for you. A large part of the playing area (not the "map") is a treeless area with container buildings, old cars, a couple of helicopters and other structures.