@SummerDCs your DC is DS or DD?
As the prep school boys mostly do 13+ entry for some boy schools. Most state school boys will take the 11+ route. Even the selection happens at similar time with almost the same content, the competition is different down to places available at different years. So for those schools, state primary boys aren't really compete with prep school boys for the place.
For coed and girls school mostly 11+ is much straightforward.