they'll just be working to a headcount
And, maybe, an arm count and a leg count.......
Sorry if anyone thinks it's a bit tasteless but it's not meant to be facetious, just a statement of fact.
As for POB vs SOB, I don't think it's particularly a UK thing but it certainly originated many years ago - perhaps in the airline world to distinguish between those who paid for their seat and who didn't. As far as ATC and SAR are concerned, though, the only thing we're usually interested in is total persons on board (POB) for the reason above.