MH is broadly broken in to Patient movements ( hospitals, nursing homes) and industrial( inanimate objects)
Would be very hard to do both as a generic MH instuctor.
As Complainer says, most H&S trainers would offer MH as part of a suite of training packages.
The big training providers often offer contract work, however its rarely risk based and they just offer MH generic course , rather then the bespoke course that addresses the actual MH issues with in a workplace.
MH training is currently been reviewed by H.S.A and likely to follow the OFA route and become more closely regulated.
I doubt one could make a living just from MH training.