As someone who has done quite a number of UK <-> Japan flights -
To get a direct fight to the UK for £550 on BA you're laughing.
Yes you can get cheaper - the cheapest I've ever had is £460 on Finn Air a few years ago, £480 or so on Luftansa so to get £550 especially at Christmas is a good deal!.
(Note a LOT of Japanese travel at Xmas / New year because it is is a major holiday season so the prices sky rocket around this time so you're on a winner there imho)
-mrploddy