Although nearly all towpaths are theoretically open to cyclists there suitability is another matter. I find the main problem is the variability of the surfacing. Some sections are well surfaced and make for excellent cycling while others are unsurfaced and can be very slow and tiring on a bike. Unfortunately I know of no map that will give this information and one has to rely on other peoples reports. This can make planning a route rather difficult - particularly when trying to estimate average speeds.
One route following waterways I could recommend is from Bristol to London. From Bristol take the Sustrans path, NCN route 4, to Bath where you pick up the Kennet and Avon canal towpath, which is well surfaced as far as Devizies. From Devizies to Newbury you can choose between the K&A towpath or the parallel route 4, this towpath section was not so well surfaced the last time I used it. From Newbury to Reading the route 4 is mostly on the towpath anyway. At Reading you pick up the Thames Path into London, with route 4 as an alternative again.
From London you could extend the trip along a number of waterways - around the Grand Union and to the North, along the Regent’s Canal and up the Lea Valley, or further along the Thames Path to the East.
To get back to your starting point you would have to take the train, but all these routes have good train services.
