The EC 261 requirement to get BA off the hook is not just that there need to be extraordinary circumstances, it's that those circumstances 'could not have been avoided even if all reasonable measures had been taken'. Given that you didn't fly until the next evening I would be arguing (even if BA's explanation of the reason for the cancellation stacks up) that the cancellation could have been avoided if the plane had been delayed overnight, rather than being cancelled outright. Just because a delay was justified doesn't mean that a cancellation was. I have had success taking that line of argument to CEDR recently.