[Arran]

The story that I read about why most British sets used a low tube/valve count had more to do with customer demand. The British Valve Association was a patent pool/cartel among the tube manufacturers like Brimar and Mullard in old blighty and had nothing to do with the government.
According to an article I read in a Radio Craft magazine from around 1946-47 explaining the design of British sets the BVA would sell tubes to the radio manufacturers at almost a loss leader price and then take their pound of flesh when the owner of the set needed the tubes replaced. Sometimes the markup would be several times what the radio manufacturer had to pay, so the inclination of most customers was to buy sets with no more then five tubes. There was also the retail price of the set that one had to take into consideration as well as what they called the purchase tax which was close to 30% at the time. Fewer tubes meant a lower retail price and of course a lower net price.
The exception to this was the British divisions of American radio manufacturers. Kolster-Brandes, Philco, and some smaller start up manufacturers predominantly used American RMA style tubes, not BVA style tubes. American surplus tubes flooded the market over there and were substantially cheaper then the indigenous types, so many of the smaller companies, and the rental radios, used those.
Regards
Arran