At the risk of making sweeping generalizations, I can only say what happened at my DC's (non-selective) prep (and what happens every year)-
There were 3 ability sets for English and Maths.
The top set were advised to have a go at LU, Hampton, LEH and, in a couple of cases, SPGS, Westminster or KCS. They were advised to sit KGS or Ibstock as "back up schools".
The middle set sat KGS as first choice, with other, less selective schools as "back ups".
Everyone got their first choice school with many from the middle set taking up KGS, but nobody from the top set.
Sorry to sound blunt, but, from what I've seen, the cohort at LU is likely to be more competitive. This can be an important consideration -would your DS thrive better at the top of his academic cohort, or not mind the competition?