Hi JSL.
Thanks for the reply, and the offer of help, but I'm more than happy to continue this discussion with BA customer services myself if I think it will achieve anything. All I really need to know is whether I am entitled to get my BA Miles back as a result of the involuntary downgrade. After all, BA Miles have no cash value so I'm unsure whether I have actually suffered any loss that a court would recognise.
(In fact, it was worse than a simple downgrade: the flight was operated "on behalf of" British Airways by some cowboy outfit called Titan Airways, who were on a par with Ryanair in terms of service and onboard ambience. The fear that BA will again subcontract their service to Titan means that I have already booked with Swiss for my next two trips to Zurich).