Do it the safe way:
On the vehicle will be a plate giving the GVW (gross vehicle weight) This is the total maximum amount the particular vehicle can weigh on the road, including everything - driver/passengers, fuel, load etc.
Fill your van up with all your gear, fill the fuel tank, take it to the nearest public weighbridge and have it weighed with you and your passengers sitting in it. The difference between the recorded weight and the GVW is how much spare payload you've got.
You get a certificate so if you get stopped you can prove you're not over the limit.