For the vast overwhelming majority of cars sold in the last 30 years, the manufacturers publish a limit. Just because that limit isn't totally supported by specific legislation doesn't mean that you won't get done for dangerous driving by exceeding it. The manufacturers have stated that, in their opinion, the car is dangerous to drive above that weight. If you want to defend yourself, you have to explain why the designers of the vehicle know less about the safe towing limit than you do.
Also, I'd guess a 'DD40' conviction would be worse on your record than a 'CU50'. Getting insurance with a DD code on your licence must be pretty damn hard.
Note that there are loads of things you may be prosecuted for whilst driving that don't have a specific piece of legislation. They are just prosecuted under a general careless or dangerous driving charge.
Christian.