Your challenge, should you choose to accept it, is to have the British military adopt the 6.5mm Arisaka round for all main service rifles. If you prefer, they can continue to employ other rounds in machine guns, airplanes, and the like. The changeover should be completed by 1950. Bonus points will be awarded if you can somehow get the round adopted as a result of the 1912 trials.
So, how can this happen, and what affect does it have on British military performance?