Our 2013 Triton 430 (in Germany) is rated at 1200kg according to both the plate to the lower front of the door in addition to the caravans registration documents. I can't advise on the law/regulations over in the UK and can only say that if you were to be stopped and checked over here (regardless of vehicle origin) the plate is the bible and if found to be overweight then it can cost dearly.
By the way, to qualify for a 100km/h sticker the axle has to be rated at 1200kg, have tyres of a certain specification. In addition the towing vehicle has to be fitted with ABS and several other items. Failing qualifying for the sticker, one example of which would be having tyres older than 5 years fitted to the caravan, then the usual maximum 80km/h speed limit applies.
It sounds as though your caravan has the bog standard axle fitted for the newer versions of the 430 and not the uprated axle. AFAIK, there will always be a discrepancy between what's stated on the plate and what's stated on the axle as there has to be a degree of safety margin the same as tyres i.e. they also state a maximum figure although day to day inflation pressures are considerably lower.