This is another unique post box, as it is the only one in London that's MADE OF WOOD!
Access to the box is only possible whilst the Royal Academy of Arts is open (check online for details). It can still be seen through the gates when they are shut, though.
The cache isn't at the stated coordinates. To find it, check out the double doorbell on the wall just outside. The number of letters in the two words above the top bell = AB. The number of letters in the two words below the bottom bell = CD.
The cache is located at N51 30.(A-1)(B-C)(D-3) W000 08.(C-2)(D+2)(A+1)
If you are interested in unusual pillar boxes, then check out my other caches in this series:
Special Delivery #1 - GCACP55
Special Delivery #2 - GCAR9WC
Special Delivery #3 - GCARPBA
Special Delivery #4 - GCAVWE1
Special Delivery #5 - GCAXDX6
Special Delivery #6 - GCB4GFY
Special Delivery #7 - GCB118Y
Special Delivery #8 - GCAXE0N
Special Delivery #9 - GCB2ZV0
Special Delivery #10 - GCB359X