Don't know if the following helps.
We have a Ds who would have been in the same year as HG's at WinColl. I wanted him to go there because although not College level ( the scholars' house) he is bright, in my view the liberal education there Is second to none, and I thought the peer challenge would be good for him.
After a grim 2 years in the local primary he transferred to a small happy but academically poor private school, knowingly. ( he was desperately life threateningly ill). He then went in year 7 to a local private school where the liberal education is not so good but the maths, his fave subject, is outstanding with more boys doing further maths than anywhere else in England.
We gave him a little extra tutoring in the term before the WinColl entrance exam so he would be familiar with the exam format. He was never going to do the Latin . But they accepted that, ditto the French, and that he is dyslexic. He got a place, but in the event decided not to take it up. I still regret it on one level, but we were not going to force him to board and this private school has turned out fine.
So
--be aware that some schools have their own entrance exams, not just CE,
--be prepared to put some effort in because your private school won't, ours didn't, why should they?
--And choose the right school for your son. Ds gave the head a hard time over WinColl football rules, and a hard time on politics, so I think would have suited the place.
And although yes academics are v v important, actually, most of Ds's cousins are there and have done heaps in the way of music or drama or debate or whatever.
I would also say, if it, whichever it is, is clearly the right school for your Ds, then go for it. But if there Is no clear blue water between it and your local one, do not kill yourselves financially. It's simply not worth the stress. Things can change v fast in rhe employment world and it's better not to have that level of stress.
Also remember that eg school trips cost. If the school has a huge endowment they may contribute. But if not, they won't be able to.
Hth