This is going back 7 or 8 years now, but when DS has a general interview at KGS they just asked him his favourite subjects and a few other chatty questions like that. They basically read his reference to him (!?) and asked how he would travel to the school. I think that school is slightly more competitive these days (as they all are), but back then, they would have had to do something very bizarre to not be offered a place after a general interview.
I have had and still have DC at LU, G&L and SPGS and academically, there is nothing between them. The same child will get the same academic results in any of them. So much is unpredictable - ie. whether they hit it off with particular teachers. But mainly, of course, their progress will reflect how happy they are and this will depend on whether they have positive friendships. This is far more important than stats such as 'SPGS 98.9% 9-7, G&L 97.2% 9-7' - or whatever the latest stat may be. These differences translate into absolutely nothing in practical terms once they are in the actual schools. Even in a less competitive school such as Ibstock (mentioned above) the same child will get the same results. Any of these schools will enable a child who is capable of all 9s or A*s to do just that.
Then they get to the other end if the school and you realise that the more A stars a school gets is actually a disadvantage in terms of uni admissions, as students are contextualised against the average performance of their school cohort. So better to get all A stars in a school where only 50% of grades are A star, then one where 90% are.
Sorry to chip in but I just wanted to say that all the schools in this area are excellent and any one will do. Good luck!!!