It very much depends on whether you are coming for a cycling holiday - that is cycling from a to b, spending a little time/overnight there and then cycling on to c, d, e etc, and if so, how much time you have, whether it needs to be a loop, and the kind of distances you’d like to cover each day - or whether you plan a holiday somewhere and want to cycle in and around that area.
It is theoretically possible to cycle over 3000 miles on towpaths, but the reality is that many are poorly maintained (a few near me are impassable presently) and/or don’t run in areas you might want to go.
The UK is generally dire for cycling infrastructure, if I’m honest, but there may well be good routes people can suggest that are a mix of off-road and quiet roads, if we had a little more information.