'sure you could enforce a unit on sen onto teacher training but realistically it's still going to be superficial because it's such a broad area and no one professional be they teacher, doctor, whatever can know everything about everyone's condition.'
Whilst this is true, a lot of it could be overcome by good training in data-driven evidence-based practice. You don't HAVE to know the ins and outs of a condition, you just have to know behavioural techniques that ensure engagement and motivation and how to recognise that in children for whom traditional and 'typical' motivations like pleasing the teacher or not making their peers cross, don't apply.
There is a lot of classroom management training, but that is about applying behavioural techniques to encourage compliance. There is very little of this in terms of encouraging LEARNING in teacher training.
Class teachers give the children tangible rewards for doing whatever makes the teaching workable i.e. stickers, golden time, moving up rockes/rainbows, allowed to take the register to reception etc. but when it comes what makes school and learning workable for the child all of a sudden children are expected to have intrinsic motivation and simply 'want' to learn.
For most kids this is fine and they can understand that there is a pay off somewhere, but many children with SEN just can't see what's in it for them.