I'll assume you're in year 12, going into year 13, and applying for 2022 entry.
You can register and book the LNAT from 1 August (2 weeks time get ready), You can then sit it from September 1st.
If you're applying for Oxford or Cambridge, you will have to sit it before October 15th. For everywhere else you will have to sit it before January 15th.
You can sit it up until next July I believe but if you do so past the two above deadlines you will be a late applicant and may not be considered. Better to do it early and get it out of the way I say.