If anyone wants a quick solution to displaying them on a Garmin handset, I have uploaded a mapset based on the Natural England GIS data (their ESRI files) to
http://www.quik.clara.net/Natural%20England%20cROW%20data.zip
Comments welcome. Their labelling could use some work, and I have already spotted a few discrepancies (top of Brown Clee hill there are some microwave towers which are definitely NOT part of the access land), but it seems reasonable.
I’d recommend installing with mapsettoolkit, in case you do something wrong (if you do, mapsettoolkit can also audit and usually fix it). This is a transparent overlay map (like contours). The area is shown as a polygon, so inside/outside are apparent, but it would be possible to add a boundary too (it started off that way). Could also be available as a (huge!) .osm file (but isn’t yet).
Released under OGL, so no restrictions beyond the normal (and fewer than OSM data).
D.