Fire extinguishers should have suitable emergency lighting adjacent (as per the emergency lighting standard BS5266) and some parties will say that you can meet this by the use of photoluminescent signage and so all your signs must meet this specification,
However photoluminescent signage requires continuous 'excitement' by exposure to light to be able to work when needed and does not count as emergency lighting unless part of a photoluminsecent way marking to BS 5266-6:1999 (or BS ISO 16069) so in reality if you need emergency lighting you would still have to provide the electrical light fittings so you might as well use normal signs.
The requirement for the sign just to identify an extinguisher is missing is only one code's take on it - other guidance suggests it's to locate the extinguisher as per the RRO requirement regarding non automatic fire equipment