Ran into this problem again today and wasted around 20 minutes on the phone while the BA agent verified she was able to reproduce the error (which she was) and then manually made the booking.

I did check the issue that 'heartybob' noted above and ensured the country was set correctly. I completely cleared browser cache and set country again just for good measure but no go.

I have sent 'ba.com helper' a link to this thread.