In the end, BA did reply through Twitter and were helpful.
When we arrived at T5, the agent asked for VeriFLY and we said that the regulations had changed. She made a phone call and confirmed and accepted us with NHS proof of full vaccination. the lady was very tired as she had started her shift in the terminal at 07:00 and it was now 18:15. But, as I have usually found with BA agents, very helpful and efficient.