About 5 years ago, KGS would offer to anyone who had a general interview. There were also 'subject' interviews and many of them would either be offers or be put on the waiting list.
LU would interview loads - like 450 - for 120 places so, unfortunately, about half would still get a no after all that. It was also said to be more competitive for boys due to less comparable school options in the area.
SPGS would interview about 200 for just over 100 places, so it was also about 50/50 at that stage (although in reality, it was only about 70-80 places left after Bute House and Faulkner, etc were accounted for).