Training is not just about the knowledge in the subject, but if you can actually put it across properly, only some of which can be taught, some is talent - I've seen some useless, yet 'qualified' trainers.
If its paperwork you are asking about, for training you would need something in the subject (a basic firefighters course could well be enough, but then again if all you did on extinguishers was a quick run through the outdated MoF Book 3, then maybe not - only you know the depth you went into practically as well as theoretical) & something proving you have skills in training/communicating.
Its all about liability you see, but certainly the fire service college think you can become an extinguisher trainer in 2 days so it can't be that difficult!