Although in certain specialist fields, particularly related to aiports and helipads there were are allowances for things like Monnex, BCF, AFFF etc in place of water, P foam, BC powder etc these do not apply across the board.
Fire extinguisher requirements are based on travel distances, specific risks and also (for class A & B fires) complex calculations. Extinguisher fire ratings are then used to decide how many extinguishers you need.
Eg a 400 m2 room would have a 26A floor rating. If you used a 13A rated 9 litre plain water or 3 litre water additive extinguisher, then you would need two extinguishers (26/13=2). But if you used a 27A rated AFFF + additive extinguisher you would only need one extinguisher as long as travel distances were not exceeded.
In this way Sainsbury's halved the number of extinguishers it had in stores by replacing fire points of two 13A rated AFFF extinguishers with one 27A rated AFFF + additive extinguisher when it decided to replace all it's BS5423 extinguishers with EN3 extinguishers in one fell swoop.
Under BS5423, Monnex extinguishers had an advantage over BC & ABC Powder extinguishers as most normal powder extinguishers achieved a maxiumum rating of 233B in 9 kilo, whilst the equivalent size using Monnex achieved 296B, which would potentially allow less extinguishers.
Unfortunately EN3 has capped the maximum fire rating of portables at 233B so Monnex extinguishers have no fire rating advantage over other powders, so numbers cannot be reduced.
The prime advantage to Monnex and the main reason for it's use is the extremely rapid knockdown of fire it has compared to normal powders (it's an impressive sight I can tell you). If you feel the risk you have is such that an extremely quick knockdown is required and a greater chance of total extinction (as you cannot partially extinguish a Class B fire with powder, it's all or nothing) then the expense of Monnex (it's very expensive) is justified.
If you don't feel you need the unique properties of Monnex a slight advantage over standard BC powder is Purple-K which uses the Potassium ion rather than Sodium and is generally more effective, but not as much as Monnex.
Even within the normal BC & ABC Powders effectiveness can be enhanced by using a different blend. A lot of "normal" ABC Powder extinguishers use a 40% MAP content, but you can get 70 & 90% blends that are far more effective (reflected in the fire ratings) for a slighly increased cost.
This is why you need to check Manufacturers data carefully as a 6 kilo ABC Powder extinguisher can have wildly differing ratings dependant on the Powder used (e.g. the Angus Tartan and Tartan + ranges, one uses ABC-50%, the other ABC-85% and there is a stark difference in ratings.
With applications that warrant rapid knockdown and high performance powders it is usually necessary to twin provision with AFFF or FFFP extinguishers/trolleys/hoses as you want re-ignition protection and powder does not cool or smother, merely inhibit combustion and a rapid knockdown is usually followed by foam blanket to seal the fuel and cool surfaces, hence UK motorsport marshall points having both Powder & AFFF extinguishers
