Annex C is not for Class A ratings, it's for Class B ratings.
It reflects the fact that Class B fires, particularly contained Class B fires in depth, require considerable skill with an extinguisher (particularly powder & CO2) to put out and even skilled operators on test fires have to chase a flash back from time to time.
Fewer industries exist today where you need to do the Class B calculations for contained fires and of those operator training is more likely (petrochemical industries).
More often than not you need to consider spill fires under 8mm, which is straightforward as the unskilled de rating factor is 10, so an 89B Powder could be used on an 8.9 litre spill, 233B 23.3 litres.
Without protective clothing it would be very difficult to put out a test fire equivalent size of Class B fire with a particular extinguisher* & the BS acknowledges this - if you are using it in near fire ratings with PPE then its deemed a specialised fire fighting appliance outside the scope of the BS.
*if you've ever done it or seen footage you'll know why - I have film of test ground and near rating specialised fire fighting if you are curious.
As to the OP - I will consider the /10 factor for class B's .Class A I stick to code except for the really small premises (where 26A would be OTT).
Whether you had a 5A 2 litre* AFFF or 34A High Performance foam I wouldn't want non specialist non PPE wearing people tackling anything beyond a waste paper basket size in the confines of a building anyway.
*although you can now get a 13A rated 2 litre foam now - the compact size of the 13A 2 kilo Powder without the mess!
