Being paid and insured to be a risk assessor not a code hugger I'd also take on that challenge!
If BS5839-1 allows agreed variations, so should 5306-8!
If quoting the RRO I would point out that provision is risk based not standard based:
13.—(1) Where necessary [implying it isn't always] (whether due to the features of the premises, the activity carried on there, any hazard present or any other relevant circumstances) in order to safeguard the safety of relevant persons, the responsible person must ensure that—
(a)the premises are, to the extent that it is appropriate, equipped with appropriate fire-fighting equipment and with fire detectors and alarms; and
(b)any non-automatic fire-fighting equipment so provided is easily accessible, simple to use and indicated by signs.
(2) For the purposes of paragraph (1) what is appropriate is to be determined having regard to the dimensions and use of the premises, the equipment contained on the premises, the physical and chemical properties of the substances likely to be present and the maximum number of persons who may be present at any one time. [i.e. a risk assessment, which wouldn't say a small upper floor off a single stair must have 2 A-rated extinguishers in every case- let's say it's a store room with 0-1 persons, are they going to use 2 extinguishers one after the other? Should they?]