Just to follow up my post #14 above...
I had not been proactive in trying to contact BA, but today I received an email from them apologising for large call volumes, and saying that I could expect a full refund to my payment card within 10 days, without any need for me to contact them. The original email said they would be in touch to discuss options, as I might have wished to use the international flights from London which are still operating, but this outcome suits me fine.