There's a very good web site all about trailers ant towing in the UK at
www.ntta.co.uk
This is what it hs to say about dimensions:
Towing vehicle under 3500kg GVW
Length
(excluding the coupling and drawbar) 7.0 metres
Width Maximum 2.3 metres
Towing vehicle over 3500kg GVW
Length (excluding the coupling and drawbar) 12 metres if at least 4 wheels
Width Maximum 2.55 metres
Length of towing vehicle and trailer combined 18 metres
Maximum overhang of load from rear of trailer 3.05 metres
I think it's the bit about the "coupling and drawbar" that means you can get away with towing boats over 7m behind a car, as it's difficult on a boat trailer to say exactly where the trailer ends and the drawbar starts! Looks like a bit of a grey area to say the least . . .
John