It is the plated weight (or mass if you wish to be correct) which counts. This has been challenged in Court, and the ruling was as stated. There is an area near to here which is commonly known as the Blidworth Triangle (surrounded by the A60, A614 and now the A617) where there is such a mass restriction. One of the reasons for the restriction was that although there are no weak bridges etc, its the sheer size of vehicles that was the problem. Two artics simply wouldn't be able to pass one another in places and there's no return. Before the restriction, there were regularly problems.
In the early days, there was the plated/actual argument. Several drivers were prosecuted based on the plated masses, and ALL either pleaded guilty, or were found guilty.
You could have the same argument over Manufacturers and Ministry masses. For instance, a design mass could be 50 tonnes, but in the UK "normal" LGV's are restricted to 44 tonnes at the moment, and the Ministry Plate will indicate this, despite the Manufacturers Plate possibly saying different. If an artic was weighed and found to be in excess of the 44 tonnes, the driver and company would be prosecuted (dependent upon the percentage over) based on that limit, not the Manufacturer's design.
Quite honestly, I think common sense prevails. If you're driving a vehicle which is designed to exceed a certain mass (i.e. 44 tonnes) and there's a sign restricting this to 17 tonnes (usually "except for access!") then it shouldn't be used. Very often, where there is a weight limit, it usually applies to a certain class of vehicle. I have found this to be the case in France. The 3.5 limit signs often (although not always) depicts a Goods Vehicle. Indeed, we recently had the dilemma with our outfit which weighs well in excess of the 3.5 tonnes, but we couldn't have got to the campsite without passing the restriction sign, as I would suggest the majority couldn't.