Oh, that is disappointing. I'm surprised. It must be very busy ex-Tokyo that week for some reason to have no availability for 6 days. It's not unusual for airlines to be sold.out on one connecting route but to hold back seats for other routes using one of the same flights. But it's frustrating when it's preventing you from being rebooked.

I think I'd also go ahead and book something now rather than wait and hope if the schedule is that booked out. Have you checked availability TYO to Birmingham, Manchester, EDI, AMS, DUB etc? Any of those or others might give easier onward journeys for you before you confirm CDG.

And you could of course try calling again. Another agent may know a fix. Or you could discuss the possibility of offloading you from the NRT-DOH flight and then seeing if that then opens up NRT-LON availability. Slim chance, but then you can be ready straight away with your other best options that you know are available, and get it sorted on the same call. Let us know what you end up with.

In this particular case, booking the original ticket with BA may have been better. I wouldn't necessarily rule out the cancel and rebook option either, if you can find a better itinerary at a sensible price.