Found Him !

Well, what do you know? That was easier than I thought.

An initial consultation with the inter-world webnet informed me that Edward VIII boxes were rare (fewer than 200 ) 
It seems there is one in St Albans, (where I lived for a year in my student days) and also Tobermory, where I had been with my folks in the 80s, both long before I had ever heard of a Royal Cypher
Birkenhead, Bridlington, or Glasgow offered closer options.
Then, searching another site, I struck gold. This box is less than a mile and a quarter from my front door, and I have walked close by it literally dozens of times without noticing it was there!

Having collected George VI just a few days ago, thanks to Daughter #1, only E II R remains for me to blip.