I don't understand this argument - whether the passenger knew the quarantine existed or not seems completely irrelevant to whether compensation is due in my opinion. Whether the airline knew the quarantine existed and still hadn't chosen to cancel the flight I would suggest could be pertinent here, but it clearly also makes sense for airlines to wait to cancel flights where possible in case the quarantine measures are lifted, so there is definitely at least a counter-argument as to why BA might need to wait until relatively close to the date of departure before cancelling some services.