No, the British player cannot buy supply with pre-game variable RPs.
On page 9, the plastic game pieces are referred to as combat units and trucks. As supply is not a plastic piece, they are not combat units.
Under LAND UNITS on page 30: “Infantry, mechanized infantry, artillery, anti-tank guns, anti-aircraft artillery, scout cars, and tanks are combat units, but trucks are not combat units.” Supply is not listed here as a combat unit.
Looks like we should have defined combat units more carefully, but the phrase “combat unit” appears dozens of times in the rulebook and it is clear in those uses that supply is not considered a combat unit.
Thanks,
Matt
