Hi Ornella, Welcome to the forum! Here is a link to the allocation profiles for the grammar schools in Bucks.http://www.elevenplusexams.co.uk/school ... n-profiles
You will see that this year's allocation for RLS went as far as just over 11 miles, and that the single-gender schools in Aylesbury were "all offered" which effectively means that all qualified applicants who requested a place were offered one. These distances can vary quite significantly year by year but if you decide to go for it, I think you would stand a good chance of a GS place (assuming your son qualifies in the 11+) if you put RLS as first choice followed by one of the Aylesbury schools. It's worth noting that SHF (the mixed GS in Aylesbury) was oversubscribed this year for the first time, but at 21 miles would also have been within range for you. It's always a good idea to name a local school on the CAF as well since otherwise you could end up being allocated one that you really would not choose if your son does not qualify or if there is an exceptionally high number of applications that year.
There is also plenty on here to help you prepare your son for the Bucks test.
Best of luck!