My guess is that the 7m limit comes from the UK towing regulations, which are not particularly clear when it comes to defining a trailer, overhangs etc. A useful extract from
www.gov.uk, "Towing with a car of van: the basics" says
"5. Gross weight of towing vehicle and size of trailer
A towing vehicle with GVW of 3500kg or lower is restricted as to the size of trailer it may tow. The trailer can be a maximum of 7m long by 2.55m wide. A heavier vehicle can tow a trailer of maximum 12m long by 2.55m wide. Exceptionally, where the trailer is specially designed to carry long loads (eg one or more boats, gliders), the 7m limit does not apply."
The last sentence is the relevant one.
I actually think that from the trailer defns, that the trailer part of my BC26 is 7m (the extension bars for the lighting board are not fixed structures), although obviously the overall length of boat + trailer is over 8m.
Peter