I know it well and have done it before IAW the OM.
One of the points taken directly:
"The number of passengers is reduced. A maximum of 50 passengers may be carried per cabin crew member (infants not included). "
We have other restrictions such as it must be approved by our Duty Manager. A safety report must be filed. Passengers must be seated in a position where they can adequately the safety demo. We can only reduce by one crew member (than usually required). Etc Etc
It's very very clearly worded to allow us to do it in unforeseen circumstances if we meet certain provisions.
And yup. UK AOC with 230+ aircraft.