There’s an excellent thread for 241 queries...
https://www.flyertalk.com/forum/brit...iscussion.html
Your question covered in the FAQs
Q: As redemption seats are released at midnight GMT, what is the best way to secure them? There is no guarantee they will be available when the BA office opens in the morning!
A: Call a BA office that is open at midnight GMT. You will then be able to make the booking without waiting for your local office to open. Depending on timezone, Japan, Hong Kong and even the USA are frequently contacted for this purpose.