This seems workable - as long as you fully intend to use the refundable ticket if your stand-by place does not materialize (and have a back up plan to get to your final destination).

Note though that there are some constraints in terms of departure time to consider when buying the refundable ticket - it needs to be the same day so that you can reenter the airside area (you mention collecting luggage), and should be at least a few hours later than the BA flight (so that even if that flight is delayed you will still have time to secure the seat and then cancel the other ticket). Ideally you should only cancel the other ticket after the boarding complete announcement, but that might be leaving things a bit tight.