The idea of the 11+ exam was actually that it would separate children who were naturally more suited to continued academic education from those who would naturally be better at technical and vocational roles in life.
I'm fairly certain that you're refering specifically the children who come from low income background as the great unfairness here. Presumably you don't mean the ones who are PP because they have parents in the military, for example. I'm assuming this from the comments about 'hard working families'.
Children who come from families with a higher socioeconomic background generally have better educational outcomes. Therefore children from families with lower socioeconomic status have a lower starting position than your child.
All the children who want to go to your grammar have to sit the 11+. The pupil premium kids who come from lower socioeconomic households have managed to pass the same exam your child did from a lower starting position than your child. Therefore they're either naturally smarter or have worked harder than your child.
These are children whose parents have a low income. It's not some judgement of how hard their parents do or don't work and how much money they do or don't earn. It's an attempt to level the playing field for children who could have loads of academic potential but have something in their lives which is acknowledged to get in the way of having that potential fulfilled.
All you can see is what you've not got. Not how great it is these other children have achieved this.
It's trying to improve the position of children who are born into a less favourable situation to fulfil their potential, which they clearly do have if they've also passed the same exam your child has worked hard for.
The great unfairness here is that these children shouldn't have a lower starting position than yours in the first place.
On a national level, we decided to try and give these children the opportunity to show their individual potential rather than their opportunities being defined by the position they were born into. They're not getting a leg up they haven't earned, they've done really well in-spite of a disadvantage that your child doesn't have.
Stop wasting your time thinking about how unfair it is you perceive yourself to work harder than their parents. It's not about you, no-one cares about what you do or think, they care about trying level up the playing field for children.