Thanks for sharing that photo, so I've gone ahead and change this to a "disused" tag https://www.openstreetmap.org/changeset/178045448 which I think is the right edit to do. 

I'm curious what the "post hoc" tool will do about this. See here: https://osm.mathmos.net/postboxes/progress/N/N19/#18/51.55641/-0.13836  Right now that showing as a green positive match, but I imagine it might pull in my update and start showing it as missing soon, and then maybe at some point it will get an update from Royal Mail which will align with OSM (removing it). 