Can OP invoke this:
G.5.16. If flight bookings for Reward travel are cancelled by the Member within 24 hours of departure, Points in respect of the Reward will not be re-credited to the Member. In the event you are prevented from travelling by an Event Beyond Your Control we will refund your Points provided you have an unused Reward, have informed us promptly and provided evidence of the Event Beyond Your Control and paid the applicable £35 per person fee.
to save the AVIOS and voucher, and rebook an AVIOS flight from TYO (WT available)? Although I'm not sure that gets you home much quicker than rebooking in the same PEK flight on the 18th,