While I am not familiar with particular BA procedures, it is my experience that on such a flight, all CC would be on duty until the dinner service was finished. Then half of the CC would take their rest, and after another 3-4 hours, the second half of the CC would take their rest. They would then be all back on duty roughly 1.5 to 2 hours before touchdown to serve breakfast.
The fact that the dinner service was not yet complete before the PAN was announced suggests that all CC would have been in the main cabin when the s___ hit the fan. Again, if BA allows some CC to take their rest immediately and not participate in the dinner service, my speculations are incorrect. But in any event, the vast majority of the CC would have been in the main cabin during the entire several hour length of the flight, so if there was a real problem, it would seem to have had as its origin something that happened prior to the flight, not during the flight.