It's actually a minimum of 11 crew on the 747 and in that case the UD must have no passengers. Otherwise it's a minimum of 12. I can't see it working with manning the flights depending on the number of passengers booked on a flight. As mentioned, a lot can happen over a period of 24 hours. The figures given on the briefing sheet for the return sector are almost never accurate in my experience.
Most airlines also seem to operate with in average 2 supervisory grades in their cabins. The CSD would have been assisted, usually, with a PSR in Economy. Apart from that, galley operator (main crew) to run the galleys. So, I can understand BA's point of view as to why they say that things are expensive. Also keep in mind that most PSRs on BA actually work in the cabin (F and J cabins) as opposed to taking care of the galley. The only PSR that actually works in the galley is in the Y cabin.