lutz, there is no (normal) transit van available in the uk with a weight greater than 3.5 ton 3500 kgs !. you may be getting confused with the larger panel van that ford sell based on the fiat ducato which as i stated has to be plated within the uk and as such the owner would need an operaters licence to use even domestically.
reading the ford website it would appear that there is one transit that would just fit the bill, unfortunately its a special order 17 seat mini bus that the owner would need a psv licence for to drive legally unless he removed all the seats but then the bus would be too light to tow the caravan so it defeats the object.
i am not too sure how it works but a good friend of mine has a number of transits, one of which is called the high cube ! its the largest panel van transit that can be purchased in this country and is just under the goods vehicle plating limit of 3501 kgs, however when he had a tow bar fitted to tow his mini digger he had to have a tachograph fitted which he has to use whenever towing but it is not required to be used when the van is solo, he was told that the van when towing exceeds the limits of private and light goods use but it is apparently still excempt from the plating rules ? we have some strange laws in this country. It appears to me that the van is deliberately kept below 3500 kgs to keep it excempt by ford but by attaching a tow ball youve turned it in to some special case, ive looked at all the other main stream dealers and no panel van that is available for plg taxation is available above 3500 kgs.