If the crew compliment was three (I don't think any BA mainline flight has 3 crew) then you cannot reduce to 2 crew. 3 on a A319/320 is the absolute minimum. Any less, you aren't going anywhere.
Forgive me if I'm wrong, but as silly as it may seem, I don't think a pilot can operate as cabin crew without some sort of extra training. Their SEP training is abbreviated and refreshed infrequently. In a reduced crew scenario however it would be prudent of the cabin crew to ensure he/she was sat at an exit.