Family holidays on canals were a feature of my teenage years. My brother and I helped Dad to fit out a 40' Narrow Boat in 1972/73. Our family best for the Wolverhampton 21 is around 1h45 with the time saved being spent in the pub at the top lock.
When my Dad specified his dream narrow boat, he settled on 55' long and 2' draft so that it would be able to go anywhere he wanted to go.
If you plan to get one, make sure you have enough time - they are a money pit if you don't do the servicing and basic painting yourself. A professional re-paint and sign writing was quoted as £7k 10 years ago. Dad, at 80, did the lot himself except the sign writing which IIRC cost him around £500. He then declared that it was the last time he would be able to do it himself and the boat was sold soon afterwards.
The boat was called Basford and he had fitted it out himself from a bare steel hull - to a very high standard.
The boat equivalent of an MOT is hard work too. Retrospective modifications can be required (Dad got very p'd off with having to drill extra holes in the boat when ventilation rules changed for example). Worth doing some serious research on this element.
I initially avoided locks especially when single handing and its still a relief to get to the end of a long flight but I actually enjoy using them now. Having said that the Canal and River Trust who took over management of canals and many rivers from British Waterways a few years ago, had a big volunteer recruitment drive. As a result many of the long lock flights have such to help you through, dramatically reducing passage times.
A 55 foot boat length will certainly ease concerns with shorter locks but gives a quite dramatic reduction in accomodation space inside given outside width of hull is limited to 6 foot 10 inches to allow passage through the nominal 7 foot narrow canal locks.
As mentioned above and with eyes open to any limitations I might suffer I spec'd a 60 foot length and was pleased it was not built a fraction of an inch more. The Salterhouse wide locks on the Calder & Hebble navigation with nominal length of 57 feet were the greatest challenge so far, but I managed them by positioning diagonally after removing front and rear fenders, but with absolutely nothing to spare. Even then I could only have managed ascending in a lock due to top gate cill considerations. I would add that with the 15mm base plate I spec'd and vintage diesel my boat is quite heavy nudging 20 tons, giving a draught of 34 inches! Yes I've been aground many times in shallow sections but as the bottom is usually mud have always managed to continue even if an occasional short tow was involved in the case of poorly dredged bridge holes.
The boat safety certificate due every four years after new is usually passed without issue if you keep your boat reasonably well maintained, with most I know carrying out their own. Bitumen blacking of the hull every three years involving a haul out can be a notable expense but hoping to do the work myself this year, Covid limitations permitting.
