“could you not have provided just one extinguisher and trained just one member of staff?.” If the answer is “well yes we could have” the defence of due diligence is gone.
But the answer could just as well be "no, because I do not want my staff to expose themselves to any unnecessary risk of injury from fire, and my company's policy in the event of a fire is for staff to "Get out - get the Fire Service out - and stay out"" the defence of due diligence is maintained.
Then we must agree to differ because I do not believe your option to only evacuate would satisfy the requirement to do all that is reasonably practicable to do.
No one has said employees should be placed at risk but The Order clearly places two new duties on RPs that were not there under previous fire safety law:
1) to reduce the risk from fire and 2) to mitigate the effects of fire.
Those duies have to be complied with SFARP and the onus is on the RP to prove that it was not reasonably practicable to do any more than was done. I think the RP would be on a very sticky wicket if he took your option.
What about people in the vicinity, your employees may be safe but if you just allow your building to burn and spread to others can you argue that you are doing everything reasonably practicable to ensure the safety of your neighbours?, and they are relevant persons.