As many of you have already stated, it is a safety regulation for all BA cabin crew to adhere to. Once all cabins and galleys are secure, designated crew report this to the SCCM. Once all reports are seen, the SCCM reports 'cabin secure' to the flight crew (receiving this is part of their checklist). Before this, a PA will be made by the SCCM "cabin, seats for landing/take off". As a SCCM, I would expect all crew to sit down immediately on hearing this, if they have not done so already. Generally speaking, BA crew are very good at safety protocols.

Regards
D1L

These views are my own and do not reflect those of my employer.