After dusk, lights on? I find flicking the fog lights looks like a brake light enough to back someone off at that time of night, just a flash of brighter red is conditioned enough into peoples brains to think brake. I tend to blatantly slow right down as soon as it becomes apparent I've got an idiot on my tail. 90% take the hint and back off, the rest either scream past or just sit too close but at a speed I feel safe to manoeuvre with them behind me.
Enforcement will tell you there's always adequate warning you're in a speed cam area, they only put cameras at points where accidents have occurred previously and its best you keep your eyes on what you're driving towards at 30+ MPH. (unexpected vehicle, bike, person popping out of side street, pavement, pub etc) & not spend so much time in your RVM you're that unaware of your immediate surroundings & SMIDSY the fixed street furniture.