Crunbs, that building wouldn't even get onto TV programme Homes Under the Hammer...
The wall-boxes I've seen located at former sub-post offices, have been fitted into the building's main structure, rather than in a free-standing wall. If that was the former sub-post office building, perhaps it wasn't structurally sound enough to have the box fitted within its wall.
The sign atop the Elizabeth II pillar box, I think it's a retro-fit, as - from what I'd found to date - those signs were pre-WW2 practice, so not fitted new onto EIIR 1952-2022 boxes.