If your emergency plan requires the use of fire extinguishers, or if the fire risk assessment (FRA) has determined that extinguishers are required to assist in maintaining a means of escape - then you do need to provide training

(Artice 21 of the Regulatory Reform (Fire Safety) Order 2005 applies

How often and how detailed the training should be is determined by the level of risk (as identified in the FRA). A single staircase old people's home with minimal staff at night would be much more of a risk than a small ground floor office.

We employ around just under 10,000 staff and do not train all of them as that would be very expensive and a disproportionate response to the risk faced. Only Fire Wardens and a Emergency Response Team get training as they stay in the building longer than anyone else. Wardens get a show & tell lecture and the ERT get full extinguisher training