I trained in Primary with a specialism in SEND. My training was a one year schools direct course, so I spent the entire time teaching in schools. For one term I was placed in a special school. My essays had to be SEND focused. I then used this to go on to be a SENCo in my NQT year and during this year I then completed my NASENCo course so I could be a fully qualified SENCO.
Tadss teaching alliance is who I found the SEND teacher training course with and this was affiliated with Exeter university. Unfortunately they only do primary training but as I am French I now am a French teacher and SENCo in a secondary school. I would say however that teaching in primary does support the understanding of how students learn the building blocks which is useful for delivering literacy and maths interventions in secondary.