This is the best website i have found on the subject of trailing:
http://www.ntta.co.uk/law/law/dimensions.aspx
It has always been surprisingly difficult to find the law on the subject of maximum width or length. I was once stopped towing a Dragon and had an extended conversation with a policeman about where to measure to and from to check a length rule he was quoting (the 7m).
My conclusion was that the Seal 26 is probably not really legal behind a 4x4 car, but was ok behind a small truck/van. There is probably a crossover point with a big or truck based 4x4.
On the other hand if we had a trailer I would not be too concerned about taking it somewhere for the winter on the local roads.... 90% of police are quite reasonable people so the only issue would be to check you were insured.
Antony