The onus is on you either to remove the towball or part of the offending tow bracket when not towing, so that speed traps, cameras, etc. can see your number plate clearly.
Some towballs are on a quick release system for easy detachment so as not to obscure the number plate, and others have the number plate incorporated into the lower bumper of the vehicle. You might consider relocating the number plate higher up on the vehicle. It may not be pretty, but it may save you a fi ne.
– Godfrey Castle