Just to clarify: councils have a duty to maintain the *signs* on a right of way. The landowner has a duty to maintain the right of way itself and any gates/stiles on it, so if it's overgrown, the landowner should be clearing it. However, if you complain to the local council, they will contact the landowner - usually a more productive method than trying to contact the landowner yourself.

The issue over copyright all revolves around the words "derived data". Under the Mapping Services Agreement that councils sign with OS, any data set created by reference to an OS base map is derived data, and the OS retains certain rights over that data. A council could not "give" its ROW data to OSM: it can only licence its use under restrictive terms determined by the OS. The OSM Creative Commons licence would conflict with these terms.

(I work for a council, though not in the Rights of Way section.)