I have shot a number of tower shoots with light english double guns loaded with the appropriately named B&P High Pheasant 2 1/2" loads in their italian #6 and #7. They come in 1 oz 1160 fps and 1 1/6 oz fps 1280 fps. Both work very well, but the latter are particularly effective. As the 1 1/6 rounds are little hotter and there are ususually quite a few rounds shot at these events, I prefer them in #7, which feels a bit softer. If I shoot the 1 oz, I use #6. Generally, if the gun is on the light side I shoot the lighter load. I like them well enough I shoot them in both 2.5 and 2 3/4" guns.
I have used the B&P #5 in the field, but most tower shoots will not let you use anything bigger than a #6 for safety reasons, and they are not necessary. Bandpusa.com will deliver them to your door with free shipping, and they are very nicely priced.
The Gamebore 2 1/2" I've shot certainly work, but recoil is nasty. The RSTs will also do the the job well and are also softer shooting like the B&Ps. Both cost more more and charge shipping.
There are now some Holland & Holland 2 1/2" available made by the famed English Hull cartridge company. I've shot some at high clay targets and like them. Not quite as soft as the B&P or RSTs, but much nicer than the Gamebores and you might enjoy shooting a true english made cartridge. You can get them at British Sporting Arms in Millbrook NY (bsaltd.com) Be sure to ask for the new Hull made cartridges, not the older H&H's made by Gamebore.
As noted previously, incoming pen raised birds are not like shooting Dakota pheasants or waterfowl, they come down much easier. So no need to unnecessarily beat up your gun, yourself, your birds or your neighbors with heavy shot and hot magnum loads.
