Caravan insurance as you have frequently pointed out sometimes includes limits on things like weight which if you exceed it the insurance may be void.
But if as the OP has stated they are exceeding the permitted vehicle weights ,
If you exceed a statutory weight or load limit, (and the same applies to speed limits) regardless of the amount you are illegal. The lawmakers do not set a tolerance on the limit, its black and white and absolute. Its the law enforcement officers that may apply a tolerance based of the capability of the measuring equipment. Because of the costs involved in pursuing a case, the enforcers will usually only pursue a case where there can be no reasonable doubt about the measurements used to prosecute.