I have been able to find out that the speed limit for narrow boats (and others) that travel on the British Canal System is 4mph maximum.
This answers the question regarding the sort of speeds these boats travel at as they slowly work their way around the canal system.
Even 4mph can be annoying for some of the boats that are moored-up as the wake (even at this speed) can cause them to rock, and this is not so good if it is either your home or your place of work.......or even when just trying to relax in the evening when watching the TV.
Bob.