I use Brit 6's (slightly larger than American 7's) on wild ringnecks over dogs, with excellent success. They tend to be the shot size of choice on British driven shoots, other than those featuring extremely "tall" birds. As Dick said, although driven/tower birds can present longer shots, that's at least partially offset by the fact that you're shooting them as incomers, with the vitals much more exposed to your shots.
RST has 7's for your Brit double. Kent Gamebore offers 2 1/2" shells with British 6's.
