You’re absolutely correct about the currency issue and that it’s specific to ba holidays but not an issue at all with ba as an airline. it’s indeed important to bear in mind when seeking changes with bah (including paid ones).

Having said that, there is no way ba mainline would have allowed a change of departure point for a 20 minute schedule change (unless it specifically made booked connection impossible) and even getting a free change of flight with them would have been a tall order (normal threshold for ba is 2 hours. You can get exceptions allowed for less than that if you have status but even then, 10-20 minutes would likely be pushing it for most agents).