If you are the sort of family that goes on Mumsnet asking about how to prepare for the 11+, your children will almost undoubtedly do well anywhere- but a private or grammar school will probably provide cultural capital that in other schools you may have to provide yourself. Research this in advance.
Don’t listen to the extreme grammarphiles either on here or in real life-you will scare yourself. See previous comment about supported children doing well anywhere.
Do not let your child set their heart on any particular school- things can go wrong on the day. And even if they do go right on the day you can’t really know what a school’s like til you’re in it.
It might feel like the most important thing in the world right now, but it isn’t!