Due to the size, I cant see smoke detectors being necessary unless someone creates an inner room in a portakabin. Even then, if the 'examiner' is stood in the access room while the 'examinee' is in the inner room I would see a good case for not bothering with detection and including that decision on the risk assessment.
Most guides will ask for a minimum of 2 extinguishers, but something the size of a portakabin could easily be risk assessed down to one for something like an examination room. (New guides mention 'small' premises as <90m2.)
My simple advice would be: Include the building in the main risk assessment, get a suitable extinguisher in, put a simple fire action notice up, make sure that whoever is accompanying the visitor is aware of the action to take in the event of fire and finally have some sort of dressing gown so that in the event of an emergency the person does not have to get dressed first. (Or indeed run out naked)