Contact the school? I think this was suggested on your other thread. They will tell you straight away. Good luck.
Edit: Slough Grammar website:
Most pupils join Slough Grammar School at the start of year 7 having successfully passed the 11+ entrance examination. We are aware, however, that there may be reasons why you may wish to admit your child beyond this point. For example, you may have moved to the area, or you would like your son or daughter to benefit from a high quality education that we offer.
Slough Grammar School is over-subscribed, which means that we do not usually have a place. However, places do occasionally become available which means we will consider new applicants for entry.
As a selective school, before a place can be offered to your son or daughter, s/he must sit and pass an entrance examination based on English Language and Mathematics. We may also request references from the most recent school your child has attended.
Langley Grammar website:
Year 8 – 11
Admission to other year groups will be subject to the availability of places, and the demonstration of a comparable standard of ability to the year group in question. This will be determined by tests set by the school, and by a favourable report from the applicant’s current school.