If the date you want has been released then I suspect it was snapped up on the day of release. End of March is peak tourist season in Japan due to cherry blossom (as well as coinciding with school holidays next year) and Tokyo flights rarely appear at good prices in BA sales so it would be a popular (& high value) redemption. I'd lock in an outbound to China & be planning on staying up late ready to call an appropriate call centre bang on midnight the day of release for your return.