https://wiki.openstreetmap.org/wiki/Good_practice#Map_what's_on_the_ground
On the ground It is mapped as (B4276) which means as stated by the "know your traffic signs" government publication "Route numbers in brackets are roads that will be reached by following the route indicated." meaning it is not that road number but it leads to that road number. Following the "Map what's on the ground" principle even if you found a contradicting source, the signage on the ground would take precedence. This is not the first time I've made this correction either. The changeset where I corrected this last has "Survey" listed as a source and  I can provide images of the signs in question on request. You overruled my changes with no sources, no discussion and no justification. 
