Are you in the big smoke now or still in Essex?
If you're in Essex tbh I'm not sure they're worth it any more as it's all yellow fixed cameras and mobile laser traps that give you no chance of escape.
If you're in london then a GPS based one will be best as they have SPECS, truvelo, digital cameras etc that aren't radar based now - and a radar detector will drive you nuts lol. (I know - I've used one in london LoL)