As I hadn't heard about this policy until this thread started one wonders whether there has been an "incident" triggering a reminder to CC to enforce this particular SOP?
And as for UniFoxOs' point re BA CC having a hissy fit, he may be right. I was irritated on my last BA LH flight to hear the CSD's welcome announcement include an apology in advance for any slippage of service standards "because we are now required to have 14 cabin crew onboard instead of the usual 15".