Having looked at the Definitive mapping for Hampshire (in order to check the current status of an apparently re-routed/obscured footpath, and report the problem, it occurred to me that it might be a good idea (and OpenData policy) for councils to publish PRoW data as GPX files. All PROWs are given an identifier, and these could be used to dfine the bounds of the GPX files.
These files could then be pulled in to mapping tools, and cut and pasted to come up with route plans that adhere to the latest definitive mapping.
The problem seems to be that the definitive map is Crown Copyright. Which, in my book, says 'public data'; gathered by public officials, paid for by the public.
It would be the obvious way to ensure that the published PROW data was bang up-to-date; the PROW data would be 'live', and updated as soon as revisions are made. It might be necessary to isolate the genuine definitive map data sets from the published GPX files, to protect them from malicious modification, but that should be covered by normal server protection measures.
If all we need is the PROW route data, that should be available independently of OS mapping.
BTW, Hampshire's PROW website is rather good. The Definitive map is available as PDF chunks.