Academic, sport, art, music, DT, drama?!
It used to be the case that if you managed to get a place at Wilsons (when it was 120 boys a year) then you would usually also get a scholarship at Whitgift, Recently it has become much more competitive and last year I know one boy who got a place at Wilson's but not at Trinity. So you would definitely need to be at grammar-level to get one these days, and even then in the top 2/3 of the grammar cohort. Depending on where you live it may also be worth looking at Reigate Grammar and Caterham as both schools seem to have increased their scholarship amounts in recent years, and, especially once you strip out the fact that they have some pupils there from 4 or so, their results seem to be on a par with the Croydon and Sutton schools.
Most prep school parents will try for academic and at least one co-curricular area in order to improve the chances, and the larger awards seem to go to boys who can offer something in more than one area of school life.