Welcome to the forum
What Mudskipper has said is accurate. Kendrick don't have a catchment, but Reading Boys does approx 10mile radius.
Whilst Slough Grammar's don't. If you are a practicing Roman Catholic then you also have the option of St Bernards Grammar, also in Slough (not sure of their catchment requirements though).
Further detailed information can be found regarding these schools in the stickys at the top of this forum and here for Berks grammars
and here for Bucks grammars
Langley last year had approx 1100+ children applying for the 145 places.
In addition to catchment, different schools sit different 11+ tests. RGS have VR (verbal reasoning) or NVR(non), Maths and English multiple choice tests. Kendrick have VR and NVR standard tests. Slough grammars have VR and NVR multiple choice. If you move to certain parts of Maidenhead, you can also then consider Bucks grammar John Hampden, where your child will have to sit two VR tests and the best of both will be the eventual allocated score.
The grammars are a good choice but not guaranteed, therefore playing it safe I would also, if in your position, look into a fallback option. Either an Independent or good Secondary State schools near by.