thermonuclear:

The oldest working box is in Dorset and has been in use for 169 years now.  Given the way letter-writing has fallen away across the globe, I'd think there will be relatively few brand new British Post Boxes going up with the King's cypher on them.  If anything, they'll become something of a novelty because they're likely to be rare. 

First CIIIR Letterbox Stolen Minutes after Install

 

"The cheeky buggers pinched it minutes after it were set up" said Mrs.Erma Fotheringay of Soggy Bottom, Z80-CPU, Yorkshire. "The concrete were barely set when ..."