Thanks again, guv1976!

I've had a look around and it appears that purely in terms of price, Aeroflot via is by far the cheapest option from pretty much every single major European airport. However, the weird thing is that Aeroflot is quite pricey if you fly only SVO-NRT - more expensive than the indirect flights from the rest of Europe!

AMS-SVO-NRT is cheapest at 371 pounds, which really is a steal for a full-price ticket of that distance - and AMS is dead easy to get to from London as well, so I think I'll settle with that even if I end up not spending my miles - that way they can go on short-haul flights around Europe where BA does reasonable rewards.

Thanks so much again for your help!