I can confirm from seven years of recent experience that one ounce of English number 6 size shot is all you need to bring down driven pheasant here in England. I usually use the Hull Cartridge company's Imperial Game loads in a 2.5" cartridge.
After Christmas I go up to the number 5 size pellets and to a 1 & 1/16th ounce load, also in the 2.5" case.
Remember that these birds are being driven towards you so usually the head is hit. Any heavier loads are unpleasant in my 6lbs 12 ozs. Webley boxlock.
HB
Hunting wild pheasants, on public land, in winter, here in the northern midwest, is truly a different game. I would imagine rough shooting pheasants in the UK is similar, with the exception of the downgrade in performance of using ammunition that is often below freezing in temperature when it is fired. Using 1 1/4oz loads is not as unpleasant as watching them fly away, leaving a few feathers to float down to the ground, when an ounce wasn’t quite enough.
I’ve read and experienced a little bit of driven pheasant shooting, but, I’d love to see a bit more on the experience of rough shooting pheasants in the UK.
Best,
Ted
