I've had a similar-ish problem today checking into a pure BA flight with a domestic to international connection (MAN-LHR-DXB). The MAN-LHR is tomorrow and the LHR-DXB the day after.

I get as far as entering how many bags to check-in and check that I understand the security question and click continue. At the next screen it tells me that check-in is unavailable. When I click on why it says that BA cannot offer me a choice of seats to choose from. Thank goodness I already have good seats on the UD!

Very odd. Never seem this before.