IPads are definitely fantastic for many kids with SEN, I think that goes without saying.
For general learning, they can be useful - but are they more useful than employing £300 million worth of teachers? Or investing £300 million in school equipment? Or any of the other things that £300 million could be spent on? Particularly when a lot of the support and training is patchy, apps haven't really caught up with demands, and teachers are incredibly busy just keeping things going.
I'm fairly sure that if my teacher friends were given £1 million to spend in their school, iPads wouldn't be the top priority.