I agree with John's comments. You will not encounter the slightest problem in obtaining the correct type of cartridges for a 2 1/2" chamber gun here in the U.K. as most loads sold for game shooting will be suitable. There are many brands to choose from. Just tell the gun dealer what you want them for and the gun you use and he will advise. There will be many brands to choose from.
Most pheasants will be shot with 1 ounce loads of number six shot. 1 1/8th. ounce loads are considered heavy here. There are some shoots that put on very high birds and there is some controversy here over the practice because of the risk of wounding. For these shoots, as John says, they will use over choked guns firing 1 1/4 ounce loads. Last year I shot driven pheasants (not overly high, just normal) with a 28 bore and 3/4 ounce loads. I had no reason to be disappointed and kills were clean.
Talk with your host. Don't forget there are beaters, flankers, stops and people behind picking up. If it hasn't got sky round it then leave it as there will be another along in a few seconds that will provide a good safe sporting shot. Good luck. Do you know where you are going yet? Lagopus.....
