Hi planforfire
I assume that the following gidance document is the one that applies to your care premises- but most are similar.
http://www.communities.gov.uk/documents/fire/pdf/151786.pdfTake a look at page 114, 5,6. This explains the requirement for training. You will see that, where relevant, you should train staff in the appropriate use of fire fighting equipment.
You have a duty to provide first aid fire fighting equipment and to train staff in when and how it should be used and who is to use it.
Your Emergency plan should set out who amongst your staff will be expected and trained to use the equipment. This could be all of your staff, staff in key roles or a few selected fire wardens. In deciding this theres a number of considerations- high risk areas such as kitchens where the right action needs to be taken in a decisive way, or carers who will have their hands full with other duties in an emergency.
All fires start small and if discovered in their incipient stages can be easily and safely dealt with. If somebody has to raise the alarm and leave a very small fire because you have a policy of fire wardens only to use fire extinguishers-eg such as a waste paper bin- it will grow and cause more damage and be a bigger hazard to the building.
So in short
Everybody needs training in the safe use of extinguishers according to their role within your written emergency plan.
The training for some may be as simple as telling them that firefighting can be hazardous and the names of the persons who have been trained to use the equpment.
For others, eg fire wardens, it may be comprehensive training with hands on experience of tackling several live fires.
Consider how many staff you have and their locations in the buiilding.
Consider the hazards associated with the work they do
Consider their specific role in an emergency
Consider how, in writing your emergency plan, you can ensure that a small fire can be discovered and quickly dealt with before it becomes a significant hazard.
Theres no prescriptive answer. Its down to your assessment of risk and your plan.