My list was getting too long, so I split my posts in two. I mentioned the pros above and here are the cons of St John's (that I found):
There are only lunch-time clubs (no after-school clubs), so you can't stay later than 16:20 (not sure if this applies to sixth formers though).
The Head is marmite. I know many parents who say that his father (who passed away before we came to the school) was a much better Head. I personally got on well with him though. He can come across as a bit arrogant, but I never had any bad experiences with him at all. He clearly cares very much about the school. Some kids/parents really don't like him though (not 100% sure why). If you go for an interview with him you get a sense of whether you will get on or not. Like I say, I personally never had any issues with him.
It's very boy-heavy. Definitely more boys than girls and given that it's a small school there are not that many girls to choose from for friendships (not an issue if you have a boy). My daughter was friends with kids from older years though and as it's a small school, everyone knows each other.
It's very traditional and strict. The teachers don't tolerate even a hint of bad behaviour and my daughter used to constantly get into trouble/get detentions. She's quite chatty and not studious though.
I believe they are strict on what GCSEs can be taken and there isn't such a wide choice, although you can speak to the Head about additional GCSEs as far as I know.
It's not that well-known. I think they pride themselves on being quite "exclusive".
At the end of the day, my daughter didn't like the strictness and the ugly uniform (which she said was uncomfortable, especially the winter uniform) and the kids in her form were all a bit too serious and different to her. But then if I were at school today, I don't think my daughter would be friends with me 😂 The wish to change schools came from her, especially as some of her outside-school friends went to the other school. I was quite happy with St John's.