lotty, you say you have a hunch that your son will catch up later on, you just don't expect he'll be ready for Reception yet at the age of four. That makes it trickier for you to decide which year group you should want him to go into (assuming you can manage to get what you want). Being unready to start school at four doesn't imply he'll be better off learning alongside younger children when he's older. It seems that it's quite difficult to have a child adjusted up or down a year later on except when it suits the convenience of the LA.
Many parents in England and Wales envy Scottish parents the flexibility they have in deciding which year group their children should join. But I don't think that's an ideal system either. It requires the parent to forecast years in advance whether their child would be better off as one of the youngest in the year group or one of the eldest. Some children's future development is all too easy to foretell even at the age of four, but most children's futures are not. And so parents are left debating which year group will best suit their particular child when he's a teenager: when should he sit exams, how will his social life be, what's best for his sporting future, when should he start university... all while the child in question is four years old!! To me this seems crazy, having to predict your child's future needs ten years in advance, but I guess that is how it is.
You could simply delay the whole question until it's clearer to you how your son is going to develop. What about waiting two or three years before sending him to school? You could wait until you are fairly sure he's ready to cope well emotionally. A few years down the line his support needs might be easier to establish, making it easier for you to choose the right school. By then you might even be able to see whether he's likely to belong in his "correct" year group or whether it's worth lobbying to get him put into the year below.
You might not be keen on the prospect of dropping a five year old into Year One without the gentle introduction to school which Reception represents. That could be hard. But would you feel equally nervous about a seven year old starting school in Year 1 or 2 without having done Reception? Older children are more mature and adaptable. They have better coping skills and a sense of perspective. I have a suspicion that if all children in this country started school a few years later, Reception as such wouldn't be needed.
Whether early schooling is helpful in terms of speech and language issues depends on the nature of the delay, I guess. My own daughter's SALT said that what she most needed was plenty of individual conversation with an adult or teenager in an environment with little background noise. Not school then! On the other hand one of my friends who'd been planning to home educate changed her mind and sent her son to school, having been advised that his speech would be likely to improve faster in an environment where people didn't know him well and weren't attuned to his speech, so that he'd have to make more of an effort to speak clearly.
This is all pretty theoretical for me because I am keen on home education anyway but in our case, I know that school would have been all wrong for my younger daughter when she was four. Now she is seven I could well imagine her starting school comfortably. Some previous issues have disappeared, her remaining needs are quite apparent, and she can communicate well.