If you really wanted to maximize the rtw tickets, you would start one DONE3 ex-OSL and use that for your US flights and to get to Japan. While you are in Japan start another DONEX and use that to return to the UK. Then whenever the mood strikes you continue on with the ex-TYO DONE3/4 -- you could do a trip or two within EUR/ME before continuing west sometime within the validity of the two tickets to the US and then to Asia eventually returning to Japan to end your ex-TYO DONE3 and picking up your ex-OSL DONE to return to EUR.