The authorities can't just say "your over the MTPLM". They have to have the evidence of weight measurement to prove it.I do take what you say ,but the problem can be the people doing the road side checks who may not agree and the possibility of prosecution if they insist on the caravan makers figure. . The problem is also if they say you are over the MTLPM they can stop you moving on even if you are right and they are wrong. It is a minefield that is not easily sorted, unless the caravan makers use the chassis limit. Some makers quote an MTPLM which is stupidly low which could store up trouble for the users. .
Besides which I did meet a couple of VOSA road side inspectors at a business event several years ago, and I was impressed with their knowledge of how vehicle weights are assessed and compared to the various limits etc. Unfortunately they didn't discuss caravans at the time. I would be very surprised if they didn't know exactly what they could or couldn't use.
You may have noticed how on some occasions there is a concerted effort by the VOSA and the Police to do road side checks on certain types of vehicle. Its probably limited to a range of vehicles based on the particular knowledge of the inspectors at the site on the day.