I am afraid it is a case of just keep trying. Because one flight is already cancelled then that helps, your booking is presumably in a state where something needs to happen to it, and therefore the ticking clock for the 3 week period is less pressing. But there are multiple reports of people eventually getting through to BAH, it does require a lot of work. I'd keep repeating the calls, using the specific BAH option on the IVR menu, as wel as repeating the Chat attempts. It does require specialist staff and they are just completely overworked at the moment, it's causing frustration within all of BA at the moment.