Every UK long distance route I have looked at on Wikivoyage tells me to buy a set of OS maps, hardly promoting Opendata or OSM.
I am a Ramblers walk leader with 30+ years of experience.
On thing I learned very early on is not to try walking using a text description. Sit down and transpose the route to a map, in the early days that was OS but these days it will be an OSM based map.
Reality is very few people using the paths which make up long distance routes are walking the long distance route but are out walking the dog or just making up a circlar route using other paths to complete the circuit
I have walked most of my local long distance route, Shropshire Way which is 325km, over a number of Saturdays by breaking it up at points where buses or trains are within reach.
Long distance routes have to visit certain honeypot locations which means at some points a path will be a member of several routes. The Shropshire Way obviously must cross the world famous Iron Bridge.