The best way to approach this would be to do a survey of training accidents and determine what the percentage / frequency / etc. of those accidents happened within the airfield boundary where the fire cover could usefully do something useful.
What percentage / frequency etc happened outside the airfield or were of sufficient severity / or relatively harmless?
Then you have to add up the cost of providing the cover and figure out if you are interested in paying the cost for the benefit.
How do you teach someone to land on a narrow grass runway that might be close by but unattended?
Remember those who are interested in maintaining the status quo may have that interest because it protects their interests, not necessarily because it is a good thing. If they have the facilities then only they can do the training....
Do we require dedicated fire cover within 5 minutes of all elementary schools as a comparison???
(I used to live in the UK, so I understand the situation and have never thought it much use to mandate fire cover for flying training )