I don't understand this either. Ready to Fly is all or nothing. In other words, if you got airside, thereby passing conformance, I can't see how they offloaded you. However the exception is if you are not at the gate 20 minutes before departure, in which case they can offload and put on a standby, generally, but not always, a member of staff.

You are probably in a good position to claim Involuntary Denied Boarding from EU261, but BA have a habit of contesting IDB down to the wire. If you were at the gate with less than 20 minutes before departure then you're on thin ice. Hopefully you have insurance for any out of pocket expenses, however you may get somewhere with a customer relations claim if BA's responsibility is open to question.