James, there is unlikely to be a stated towball weight for a UK caravan. There has been a guide figure of about 7% of the vans maximum weight but with larger/heavier vans this is unachievable. Most UK supplied vans have chassis and hitches rated at a maximum of 100kg. Some Continental vans have the wheels located further aft, usually by using a longer "A" frame and this facilitates lighter nose weights down to about 4%.
A good guide figure is to work close to the limits dictated by vehicle or caravan, whichever sets the limit. Its also very unwise to work with levels as low as 50 kg with a normal sized UK manufactured caravan.
Many are restrained by their tow vehicle to a limit of 75kgs and this seems to result in nice stable units, where the van is not a long one.
The towball weight is indeed dictated by how the van is loaded, and thus this is how you control what it is. Don't forget however that the biggest masses and bulk of the mass must be located near the axle as this reduces the moment of inertia of the van swinging about the axle line.["snaking/yawing"]
So with the big masses in place near the axle, logically one side of the axle or the other to get towards the right nose weight, then trim with moving one of these items either a bit one-way or the other. It soon becomes evident what you need to do and where things travel.