Trouble with RM/GPO whatever you want to call it is that it's been asset stripped for as long as I can remember. I worked there as a postie for about 6 weeks during the mid 70's recession. There were no jobs to be had elswhere, despite that few people wanted to get up at 4,30 in the morning. The wages were crap but there was unlimited overtime. Coincidentally, I worked at Slough GPO when Alan Johnson worked there although we never met. Many years later we exchanged a few emails after I wrote him complimenting him on one of his books, turned out he knew several of my mates dads who were posties. Nice bloke, I've always said if he had run for PM, I'd have voted Labour again. (yes, again, I was a bit of a socialist in my younger days).
Anyway, back on topic, first the telecoms part was taken away, then opened up to allow competition to tender for postal deliveries, obviously the competition only tendered for the big cities.
I've no idea how much a first class stamp is these days, whatever it is, as Carman said, it's not enough to cover delivering a letter to an isolated cottage on a remote Scottish Island.