There is a strict deadline for registration for the Slough consortium test. There is a small window, typically from the start of May to the middle of June each year, when parents must register for the test on the official website. Prior to this, in April, there is an online event for parents which gives information about the test. Each school offers an open evening in the Autumn Term prior to registration, so interested parents can visit the schools to help them decide.
You can learn more here:
https://www.slough.gov.uk/school-admissions/apply-school-place/8
You can see the registration portal here (NB – this will close in late October every year):