I've come up against the same problem. Myself in 1K (BA Gold) and companion behind in 2K (BA Silver). 1A was showing as available but MMB wouldn't let me allocate my silver companion that seat. A call to telephone reservations didn't help either, they reported they couldn't assign 1A either but weren't able to offer an explanation.

This thread explains everything - BA have a ridiculous policy of denying access to 1A/K for Gold members' travelling companions.

This makes having access to 1A/1K completely useless for me. I've e-mailed AskBA but no response yet. Today, alas, 1A looks like it's taken. So whoever is sat there will have to put up with me shouting back to 2K, when they might have preferred 2K themselves if they're a solo traveller.

I get the impression I'm not the first to have had a moan about this. I thought once I attained Gold status this kind of nonense would be a thing of the past, but alas no. It doesn't do BA any favours.