Good think it worked out. If BA had refused to provide me a club seat and one was available I would have paid be CC and done a chargeback.

That's what I did years ago when DL downgraded me on part of my trip; the aircraft went from a two class to all Y and then back to having an F cabin. Agent wouldn't upgrade me but at the airport offered a paid upgrade.I know for a fact that on some airlines pay a commission to employees who sell upgrades so this is probably why the agent wanted me to pay. I paid and did a chargeback which the airline didn't contest ( why would they?).

I understand that during IROPs you might be downgraded however refusing to assign a business class sea that is available to passenger who is lawfully entitled to is basically fraud and in such cases both the airline and the employee who refuses to give you what you paid should be subject to a pretty massive fine.