My guess is that the computer offloaded the OP before any human being ever looked at the booking.

IF feeder flight delayed THEN connection no longer possible NEXT offload the pax

Unfortunately doesn't take delays on your next flight into account. If that flight was oversold, they may have already given your seat to someone else so nothing they can do at the gate either.

BA will (successfully) claim weather as cause of the delay of your first flight and argue that you were justifiably offloaded. This I give little chance for compensation.