okay, but, lhr-hnd is still the most convenient and direct option from the UK. It’s not like Tokyo has poor domestic onward connections either.

theres also no other option to japan directly using a ba241 voucher. Your only alternatives are madrid to tokyo narita (much cheaper avios and cash wise) or to fly on a BA 241 to hong kong, beijing etc and connect there onto a regional cathay pacific or JAL service

alternatives to the avios241 voucher are Jal / finnair redemptions from europe / helsinki respectively.

focus cities for JAL from europe are helsinki, rome, frankfurt, paris (cdg) and of course london