I had a very similar BA seating issue last month for LHR-OSL. I initially tried to resolve it via telephone but gave up waiting. I was then able to use BA’s chatbot feature to get through to a live agent and had the issue resolved in about 10 minutes. For me, the chatbot box comes up at the bottom right of the screen on the ‘Help and Contacts’ page. I just kept entering the word ‘agent’ until it connected me. My first attempt didn’t work so I disconnected the chat and started again. On my second try it worked like a charm. I was number 2 in the queue and the live agent came on after a 5 minute wait. After providing some verification info the agent selected the seat for me. I am unsure of the hours of service for the live agents. It was about 6am UK time when I did this and I was using an iPad from Singapore. It was all quite painless, despite my 2 finger typing speed, but YMMV.