I don't picture the B.A. accepting anything commercial. If they like the ballistics of the .303 rimmed, I'd expect a .303 rimless to be very close in performance, unless it's influenced by other factors (like a desire to reduce ammo power or felt recoil or something). That's really a separate issue--unless that is why you get a semi-auto in B.A. service in the first place...does Britain adopt an existing or retool/invent something domestic?