From Historic England:
“From 1857 wall box-type post boxes came into use for fixing into existing walls. In 1859 an improved cylindrical design of pillar box was created for standard use nationwide.”
Ruth Penney wrote in Brentford & Chiswick Local History Journal 6 (1997):
“In the W4 postal district there are 63 pillar boxes, seven wall boxes but no lamp boxes. Thirteen of the boxes date from the reign of Queen Victoria, twelve from Edward VII, twenty-three George V, three George VI and nine from Elizabeth II.
The oldest of W4’s seven wall boxes is the Victorian box at the entrance to Turnham Green Station” (see image).