The standard English 12 bore game load was for years the 2 1/2" cased 1 1/16 Oz load. (Typically Eley Grand Prix, though there were many brands).
One ounce was available (Eley Impax) - as was 1 1/8", though for that you needed 2 3/4" chambers - which are rarely found in older standard English 'game' guns, being normally found on so called 'live pigeon guns' and wildfowling guns, for which 1 1/4 Oz loads were also used.
Most UK game shoots today you see a mix of 28g, 30g and 32g used (1 oz, 1 1/16 oz and 1 1/8oz). Most people shootiong English s/s guns will use 28g or 30g. 32 are usually used in the heavier o/u guns such as Beretta and Browning etc.
I was always brought up that should you wish to shoot more lead, then you used a 10 bore - or 8 bore - or even 4 bore.
For smaller gauges, e.g. 0.410, the standard cartridge was the Eley 'Fourten' which was a 2" case with 5/16 oz shot (still available today) and the Eley Fourlong in 2 1/2" cases carrying 7/16 Oz (also still available) provided your 0.410 had 2 1/2" chambers. There are also 3" 0.410 cartridges, but English guns are not chambered normally at 3" in 0.410.
