It is interesting BA have chosen the term Cabin Crew - which from a flight time limitations point of view makes zero difference being one down and returning to base. If they’d used flight crew they’d have more credence in their retort. However as you’ve already identified it was a pilot or flight crew member they were short of which does indeed have an impact.

I think you’ve gone as far as you can go with this as from my understanding it is not generally expected that airlines have standby pilots in each destination of the network, quite often swaps with other crew on different line so of work are available (subject to FTL limitations) but at more remote destinations, this is often not an option. We all get text messages from time to time asking if anyone is on a trip or happens to live in x part of the work and available to operate a particular flight in order to keep the show on the road.

If you did want to continue it would have to be CEDR but I wouldn’t hold out much hope, sadly it’s one of those things in life but quite correctly your downgrade and paid seat refund should be settled. Being stuck ill downroute really is not an option any of us want and I’ve been there myself, we want nothing more than to be home.