I must admit I laugh at the parsimony regarding shotgun ammunition displayed by people who will happily drive 100 mile round trip in a Range Rover and pay 700 to shoot driven pheasants with their 15,000 gun. In truth shotgun shells are at a historically low cost and a historically high quality and choice.
BTW - that is not a dig at people who like to re-load. Do what you like in your spare time.
Could be that 2 1/2" shells are relatively cheaper in Great Britain than they are on this side of the pond. But driving my wife's relatively inexpensive to operate Honda CRV to the range and shooting up 4 boxes of shells, the difference in price between my reloads and factory 2 1/2" shells pretty much pays for the gas I use and the targets I shoot. As I said earlier, I really like RST's (and Kent Gamebores) for hunting, but then I won't shoot a flat of shells in an entire season's worth of "rough shooting", while I might go through that many in a week on the range.