Caroline Blomfield wrote for the Kew Society in May 2020:
“North Road: under the railway bridge. This is one of only two wall letter boxes in Kew and it is was manufactured by W.T. Allen & Co, London. It is on the wall right beside the pretty ‘High Park Corner’ garden, planted and maintained by the Kew Society. The crown is just above the posting aperture, and the words ‘POST OFFICE’ on it.” (The royal cipher is GR.)
NB: this is in fact the road bridge which carries High Park Road, the route between the B353 and the A205.