For most courses starting at universities in the UK in 2024, you should register through UCAS by 31 January 2024.
UCAS stands for Universities and Colleges Admissions Service. All applications to UK universities are made through this service so, to become a full-time undergraduate student in 2024, you will need to register with them.
If your completed application is submitted by the deadline, it is guaranteed to be considered.
For any course at the universities of Oxford and Cambridge, and for most courses in medicine, veterinary medicine/science, and dentistry, the deadline was 16 October 2023.