I attach a reply on a similar subject from the C&CC forum posted by an Officer involved in checking caravans. This does not agree entirely with the answer from the Prof and I certainly thought that it was illegal to tow if the maximum laden weight of the tow car and the MTPLM of the caravan exceeded the gross train weight whatever the actual weight of the caravan, I was sure I had seen this on the CC web site but could not find it again when I looked. In this case I suspect that the actual weight of the caravan would have to be used as getting the MTPLM without a plate on the caravan showing this might be very difficult for the Police. Anyway read on:
Hi, if it helps I can outline how the UK agencies handle such matters with trailer weights. As I have worked alongside the Midlands motorway traffic police and (formerly) VOSA officers with operation Velopy.
Firstly the patrol needs a reason to pull you over, this could be a lack of towing mirrors, badly/incorrectly displayed registration plate and numerous other similar things.
Once stopped the police run a series of checks on registration, tax, insurance, licence and CRiS etc. The VOSA team look at the integrity of the combination. They also check the VIN for the weight limits and then weight he axles in turn. The sum of the axle weights must be with the train mass limit on the VIN and no axle or tyre is to be overloaded. (this applies to B+E licence holders, B licence as mentioned are capped to a maximum combined mass of 3500kg). If the weigh-pads suggest overloading they can recommend an adjustment of the load to improve things, if they believe it is grossly overloaded they have the power for a prohibition order or have you escorted to a fully calibrated weighbridge for possible prosecution evidence gathering (rare thankfully).
Thus from the above process you can see that a caravan that has an mtplm just over the towing limit may be acceptable as long as you're prepared to under-load and you have a B+E licence. With a B licence it is what's on the VIN plate what counts.
We strongly recommend you use Towsafe for matching, it is the most comprehensive database and there's a real human being behind it trying to keep it as good as possible.
For those who're struggling understanding the difference between B licence and B+E we have a very comprehensive datasheet and flowchart right here .
As for the above comment, "For instance, although either of the big clubs suggest a 80% towing weight figure, there are quite a few 4x4 manufacturers such as Land Rover who suggest you can tow at well over 100%."
We recommend an 85% weight match for beginners and to never go over 100%. As for towing limit, this is locked down in the type-approval process and I don't want to bore you with that and how some car figures are so high and other so low.