Welcome to the forum.
I'm not an expert on these areas, but there are some historical data on the Slough school
here, and Reading
here. You should carefull check the admissions criteria for each school as well, as they can change each year. Some schools require you to be resident for a time before applying (I don't know if this is the case for Slough or Reading, but it will be in their admission criteria).
I don't know the Wokingham schools at all, but I'm sure someone will be along soon who does (note that they are all state schools including the grammar schools).
If you were in Slough you may also be in catchment for Burnham Grammar, although this is in Buckinghamshire (& so involves a different exam), it is right on the border with Slough & takes a fair number of pupils from Slough (althought the distances have been shrinking in recent years). You can check Burnahm Grammar admissions information for the last few years
here.
You will also need to find a primary school place when you arrive too, so that may affect your choices (you won't be able to apply until you have an address here).