During the Postal strike of 1971, many companies,Dealers,offices etc ran their own Postal Services. Many saw actual postal use, some didn't. A good number were issued only for collectors [Which is what the Postal services are doing today
The Pirate Post of Bellerby-Palmer saw actual postal use. The comapnies themselves organized delivery. In this case, Bellerby were the Printers but I have no information on the Palmer half.
There was a long-established Coal Merchant in Exeter by the name of Palmer. He worked from the railways. So what if he saw a good way to earn a fast buck, met Mr Bellerby in the Railway Tavern on a cold January evening and there they cut a deal.
They ordered one of their trustworthy young workers to take the train [Which was probably free] first to Bristol and then to London.
Londonbus1....Well, it's possible isn't it?
PS. Phil's explanation is more likely !