I saw that Sally-Anne has answered for Bucks. Bucks has excellent grammar schools, but allocate places according to where you live. If you like a school in Bucks, you have better move into the catchment. It is different in Berks where places are allocated according to the exams results(although Reading grammar has a catchment area)
This means that if you leave in Bucks close to the Berks border you have access to both areas. (but as Sally-Anne mentioned, more preparation work for the tests).
The Bucks/Slough border is good for access to London with the M4 and M/A 40, and would give your child access to the Slough grammars, Burnham grammar(in Bucks), and the Chalfonts community College (Bucks)that Sally-Anne mentioned.
A few miles North towards Gerrards Cross (Bucks) you would also have access to the excellent Royal Grammar School in High Wycombe and Dr Challoner's.
From Slough, the rail services to London go to Paddington station, and from Gerrards Cross to Marylebone.
There are 4 grammars in Slough: Langley Grammar (Foundation - co-ed.) 145 places. Mathematics and computing specialist http://www.langley-grammar.slough.sch.u ... ssions.htm
Herschel Grammar (Foundation - co-ed.) - 120 places. Technology specialist http://herschel.slough.digitalbrain.com ... page/home/
St. Bernard's Convent Grammar St Bernard's Catholic Grammar School (renamed because the convent part has gone)(Catholic, Voluntary Aided - co-ed.) - 120 places Humanities specialist http://www.st-bernards.slough.sch.uk/
Slough Grammar (Foundation - co-ed.) - 145 places
Language + Sciences specialist
Reading grammar is a very good school.
If you trawl through this section, you'll find quite a bit of info about each of these schools.
Hope this helps.