There's a difference between having a statement of educational needs with individual funding implications and being on the SEN register at either school action or school action plus stages.
Much depends on the individual school, local education authority but overall, it should be the individual needs of the child that govern whether the help comes from within the SEN budget or is extra as the result of a statement.
If you look for threads on other major UK forum you will find a few children with statements but they often have more than 'normal' T1 needs.
I agree with Hana that DUK are the best port of call for advocacy advice.
Meanwhile this document from Children with Diabetes gives some information (2007). I couldn't find a more recent one
http://www.childrenwithdiabetes.com/uk/ ... ool-UK.pdf
unfortunately, some of the links are out of date.