Per this thread
http://www.flyertalk.com/forum/briti...e-centres.html, you can call any office you find open.
Originally Posted by BA
From today, Executive Club Members can call any service centre to make or change their bookings. This includes both tickets booked with cash or with Avios. You will no longer be asked to call only your local service centre.
The numbers that you call will not change and we do encourage to call your local service centre when you are in your country of Membership but we realise that there will be times when you need to reach us wherever you are and our colleagues will be happy to help.
I realise it's a pain working your way through the list, but are there really no other offices open ?