My EXDP was sent to boarding school at 5 years old (not in the UK).
Although he doesn't think so, I can see it scarred him for life as he has a very detached attitude to everything, and from what I've seen, he has difficulty forming lasting relationships, and in believing in them. He is extremely independent, so despite me wanting us to do things like have a joint account (I earned more than double what he did), he refused, saying we should manage our own money, then was resentful that I had so much more.
He used to ask me to read to him, which I realised was because he'd never had anyone do that. He also used to wake me up in the middle of the night (like 1/2/3am) for sex, when we had babies, plus he knew I needed to be up at 5.30am for work, and despite me asking him not to do so many times.
He was also extremely jealous of my intact family, the fact that I saw my parents almost daily, and that they would help me with the DC when he refused to. He would moan that they were our DC and that I was behaving like a child in letting my parents help us with things like the odd day of childcare or school/nursery drop-offs, yet wouldn't do those things himself as he wanted me to, on top of my busy full-time job.
I don't need to send my DC to boarding school, so wouldn't. Although I can see how it may benefit much older DC, e.g.at secondary school age, personally I think 7 is much too young.