If I remember right from previous post I think HABS just select the best from the intial test and interview and award them scholarships.
Its as long as a piece of string what level your child needs to be. We are walking that route too but a child can be awarded scholarship by simply excelling in one subject like maths or even writing so I dont think its a one size fit all. Habs and City are some of the most highly competetive schools I have seen on this forum. To just get a place is hard enough, if you use the search function on this forum and search for "habs" or other schools you will find previous post.
Also it may be worth checking the bursary options for a reduction in fees. Some schools dont require you to have passed a scholarship to be awarded one but the maximum income threshold tends to be around £45K household income.
This thread here may be useful viewtopic.php?f=31&t=12197&start=0