Hello, 
Andy from OSM's Data Working Group here.  Re "A footpath is a footpath regardless of width of path" - unless the imagery is severely out of date, https://www.openstreetmap.org/way/48131545 should not be a "highway=footway" - it's clearly an agricultural track.  It should however be a "designation=public_footpath", as it's actually also designated as a public footpath (as can be seen at e.g. https://map.atownsend.org.uk/maps/map/map.html#19/51.77162/-2.33870/H/P ).
I'd expect that the basic tagging for this would be:

highway=track
foot=yes
bridge:name=Stonepitts Bridge
surface=dirt
designation=public_footpath

If bicycle or horse access is permitted then I'd also add bicycle=permissive or horse=permissive

Best Regards,
Andy
