The very first thing I would like to tell you that please do not get nervous or stressed out as you might pass this on to your DC which is not good for him/her
I know it's very difficult but that's the best way you can handle the whole process.
Langley Grammar School which is under Slough consortium has got a clear catchment policy (you can find the postcodes which are covered by the school). I have talked to the school before and they clearly mentioned that every year they have more students with cutoff marks from the designated area than the number of seats in the school so they have never gone out of catchment.
Herchel and Upton Court do have some seats for OCC but don't know the details, someone in this forum might be able to help you there.