AFAIK there is no law about motorbike width. The main thing would be whether it is safe and not a danger to other road users.
Section 40a Road Traffic Act 1988, using a motor vehicle in a dangerous condition. The offence covers carrying any load where “the weight, position or distribution of its load, or the manner in which is secured is such that the use of the motor vehicle or trailer involves a danger or injury to any person.”
I imagine that the rules in other European countries are similar.
EDIT: There is a rule about motorcycles towing trailers that states they must not be more than one metre wide, so that may be relevant.