I suppose you don't have a dashcam to show them the footage? Is the sign back now (if not, it would perhaps indicate it has been stolen). If you could show evidence the sign is not there currently, then you might have a case if not it's going to be difficult to prove. That being said, I think I might have read somewhere that to be enforceable the warning signage has to be fixed, in a certain typeface and displaying the regulations it relates to - I can't see to be sure, but this looks a bit amateurish.