Prior to the HTM 05 series the scope of Firecode was mainly hospitals. However, the term hospital was nevr defined although many took it to refer to premises that either incorporated wards or where anaesthetic was administered. There's nothing preventing the use of any of the Firecode guidance to premises presenting similar risks.
To my knowledge, you won't find a fire safety document that refers to PHE where anaesthetic is administered. However, I think the architect and AI are on dubious ground if they try to apply purpose group 5 to this type of premises. Professionally they must consider the use of the premises and the suitability of the fire precautions to protect the occupants of the premises put to such use, and I'd certainly advise that the "assembly buildings" classification does not cater for anaestetised patients and probably not for the increased proportion of less able occupants. After all, it's mainly people with illnesses and ailments that go to medical centres!!
Reference document or not I'd stick to my guns. The risks are foreseeable and both the architect and the approved inspector should be addressing them.