I would say 'No'. British 20 bores have a standard load for the 2 1/2" cartridges of just 13/16th. ounce and are quite mild in pressure against U.S. made ammo. Most U.S. ammo is designed to fire in semi-autos and the pressure needs to be higher to cycle the action. You would just be straining the gun beyond what it was designed for. In the 1970's Winchester cartridges were imported from the States and it was found that the popular Winchester AA clay round with 1 1/8th. ounce actually approximated what was only suitable in English guns proof tested to the 2 3/4 inch Magnum proof. Our Winchester stuff is now made in Italy to CIP standards. I still have a few boxes of the old U.S. made Winchester 'Upland' but will only use it in continental guns to 900Kgs. proof. Your option but not what I would do myself. Lagopus.....
