These websites can be helpful to understand the SEND system in England.

IPSEA

Special Needs Jungle

DIASS
SEND Law and Statutory Guidance
The Children and Families Act (2014) outlines the law surrounding SEND
This is a short guide to Part 3 of the Children and Families Act (2014), the section to do with SEND.
The SEND code of practice: 0 - 25 (2015) outlines the statutory guidance for local authorities and schools.
The government published this guidance for parents.