Agree with above, only way to guarantee that staying in the year below is permanent is to get it written into the statement, at AR maybe. You could call an emergency AR. It will mean still leaving school/college at the 'normal' time, though.
Some primary schools that aren't oversubscribed will happily, informally teach a DC in the year below, but they aren't allowed to deny a DC of the 'correct' age a place because your DC is in the 'wrong' year. This is only really a big problem in infants because of the infant class size rules.
The SS that my DS went to was an early years SS. In the past it regularly had DC spend their reception year in the SS, then repeat their reception year in MS. This was before the infant class size rules came in and also during the transition time of that law, when exceptions were still allowed. These DC often were on SA+ only and they were allowed to remain in the year below throughout their school career. It worked really well and often meant that their difficulties were a lot easier to deal with in the early stages. After a few years it makes little difference, though.
LAs often come up with reasons against being behind a year, such as:
They might get teased for being older. (Small problem in the grand scheme of having SN, IMO.)
They may develop and hit puberty much earlier than their peers. (So what, there's already more than a year's variation in that anyway.)
They will miss out on a year of schooling at the end. (Up to you if that bothers you.)
Good luck. 