I can't speak for all schools but Alleyns explicitly targets a 50/50 split between state and prep schools for external candidates. They also have guaranteed places for their own prep school so the split ends up 33% Alleyns Jr School, 33% state school and 33% external prep school. Schools who do have a target split I find are fairly open about it so it's worth asking the registrar for the schools you've applied to if interested.
Prep school should an easier route to private senior school otherwise, why on earth would anyone pay for it. It's not an insurmountable advantage but like for like would require more effort at a minimum to prepare to the same extent. I say this as someone who has done private school from the start!
I can't really see why discussing this is controversial.