@Robi if by "blue card" you mean the free Amex BA card then you can only redeem it in economy. Given cash prices at the moment (seems to be over £1k return), you can still get good value though. An off-peak return to Tokyo is 70,000 avios + £200 (other avios/cash combinations are available), so by using the voucher it will cost you 70,000 avios plus £400 for 2 tickets.
Bear in mind that Tokyo is one of the most popular destinations, and blossom season is the most popular time to go, so there might not be any reward seats left for March/April 2025 if you're not booking until June 2024. If you can trigger the companion voucher in April 2024 and book then, that would be optimal