In response to your specific question, and finding myself in a similar scenario, I would have been inclined to act exactly as you did.
It’s a great shame that BA signally fail to observe the spirit of the EC261 regs in not following a more pro-active approach in such circumstances.
Management are of course all too conscious that the most ‘effective’ time to notify the legal rights & entitlement available to any passengers not already fully aware of them (and, as you say, that is almost certainly going to include the vast majority) would have been whilst they were ‘contained’ in one place - ie all sat together on board the aircraft. Hence the absence of any internal policy to ensure that crew do exactly this. The fewer the number of people who know .....the lower the financial impact for the airline.
I’m almost certain that BA are not alone in this unhelpful approach to EC261 ; but it does nonetheless say a good deal about their corporate culture where customer service issues are concerned.