Average speed in London is what, 12 mph? You'd fine people travelling at the dead of night, but that would be about it.
It's not about average speed over a whole journey. You'd never be able to speed because you wouldn't know where the next camera was. It's not possible to game a system like that by just slowing down for one camera. There'd be no point whizzing up Mitcham Road at 40mph because you'd never know that you wouldn't get photographed on a side street later and be found to have got there too fast. It's more of a constant, background deterrent rather than a speed camera, which only has an effect for 50 yards and quickly gets remembered by drivers.
It's not about average speed over a whole journey. You'd never be able to speed because you wouldn't know where the next camera was. It's not possible to game a system like that by just slowing down for one camera. There'd be no point whizzing up Mitcham Road at 40mph because you'd never know that you wouldn't get photographed on a side street later and be found to have got there too fast. It's more of a constant, background deterrent rather than a speed camera, which only has an effect for 50 yards and quickly gets remembered by drivers.