Have you thought about getting the BA amex card? With a 241 you'd get to Tokyo in J for 180000 avios plus about £1k in taxes. Admittedly you'd have to spend £10k on the card to get the 241 but given you're looking the best part of a year ahead it shouldn't be difficult. The signup bonus on the card's 30000 avios so that'd give you enough for the flights. If you need a referral for the card let me know