Unless pedestrian access is explicitly prohibited with a traffic sign and associated traffic order, it is not illegal to walk on any public road in the UK. Access tags in OSM reflect the legal position, not an opinion on whether or not it might be safe or a good idea.

https://wiki.openstreetmap.org/wiki/Key:access
https://wiki.openstreetmap.org/wiki/Road_signs_in_the_United_Kingdom#625.1