I think there are 2 issues here.
First - Health and Safety being what it is, should (my opinion) instruct people not to use the fire extinguisher if untrained. After all it is their role to ensure the safety of persons, and people should be instructed to first make all persons aware of the fire, and the H&S "global" remit, is to safely get all people out.
Even if a fire is going to be extinguished by a single extinguisher, by the discoverer, I think we would all agree that an evacuation is necessary. By instructing all untrained staff to Not focus on the Fire, but to raise the alarm and get out, H&S are doing their Job.
Secondly - In a land where the Pay as you Go Lawyers are on many high streets, companies have to protect themselves, (Sad but true), from poor use of equipment, or equipment failures, or tackling a fire too large, or any of many other reasons, it is therefore unrealistic to expect them to say, have a go if you think you can handle it. I am aware of many offices staffed by (and I do not mean to offend) petite ladies in heels, who may have difficulty wielding a standard fire extinguisher. Is it safe to halve the size of the extinguisher and tell them to make 2 trips?
Hopefully common sense will prevail, and the 70% of fires dealt with by a competent, and possibly untrained person will continue to be dealt with in the same manner.