If you've got the car hitched up to the caravan but only the wheels of the caravan on the weighbridge, you're not measuring the weight of the caravan but only the axle load, i.e. the towload.You're still getting it wrong, Klarky. The total weight of the van (1259kg) is the sum of the axle load and the noseweight. By definition, the towing weight is the axle load. The noseweight is treated as part of the payload of the car not of the towload. Otherwise you would be counting the noseweight twice.
Therefore it is not illegal to tow a 1259kg caravan behind a car with a max tow of 1200kg (so long as the noseweight is equal to or greater than 59kg)! Believe me!
If the caravan is standing alone on the weighbridge including its jockey wheel you're measuring the total weight of the caravan.
That's the difference.