I use Hull Imperial, and prefer the 26 gram version, but they are available in 26, 28 or 30 gram versions.
Do please check if your host requires felt/fibre wads as many UK shoots don't allow plastic wads (and don't get caught out by arriving with plastic if it is not allowed on your host's shoot). Personally, I would always use felt/fibre (even for clays) as I dislike to see the plastic litter. Imperial game are all fibre.
http://www.hullcartridge.co.uk/impgame.htmMuch depends on your gun's weight. A gun below 6 1/2 lbs will be very lively with 30 gram.
Another factor is the height of the birds. Some of the (few) very high bird shoots (think Devon valleys) may not be so well suited to a vintage gun with suitable loads as many of the attendees there claim a heavy extra high performance load is needed. Attendees there tend to use their 34" barelled Perazzis and 42 gram loads. I can't really comment as I have never been.
Hull Imperial 26 or 28 gram are fine for the normal shooting I do.
I hope you have a really enjoyable time with your vintage gun over here.