There are no formal training programmes to become a consultant. You need the ability to work on your own and some experience that is marketable.
Many large organisations such as the National College for School Leadership employ people in particular roles similar to consultancy. Traditionally, in education, consultancy is seen as something provided by those with expertise. This is not the same as in the industrial and commercial world where consultants offer some of the same sort of functions Ofsted inspections provide within the education world. Your best bet is to talk with some existing consultants about the pros and cons of their work.