My ds has just entered year 11 of a small non-selective, unfashionable independent school.
The key benefits for him when he goes into the 6th form will be:
Small class sizes - no more than 8 or 10 for A'level Maths, Physics, Chemistry & DT. Regular teachers who can spend time getting to know him and how best to support his learning.
Good careers advice and knowledge of the university admissions system, here and in overseas.
Extra-curricular activities that help him manage exam pressure
The pupils at his school are a mix of army kids, local professional & farming families, a very small overseas group and some like us, on a part-scholarship and scraping the rest of the fees together somehow.
There are a few well off families, but they are not the majority. The parents carpark is usually full of muddy old landrovers, elderly hatchbacks, a few Audis and Volvos (and one pristeen Maserati that parks a long way from me 😂)
If the main benefit is supposed to be social, then my ds has failed spectacularly. He has three mates who like chess and martial arts, and he sticks to them.
But he's happy and doing well academically which is what really matters to him.