UK higher education institutions (universities and colleges offer 2 intakes which is can be interchangeably referred to as semesters. At some universities, an additional intake may be available, however there are limited courses.
Which intake you should apply will largely depend upon the availability following factors:
It is always better to apply once you are fully prepared with all the required documents and transcripts in addition to being mentally ready. Individual universities and individual programs may have their own specific deadlines for international students applying from abroad in order to facilitate or allocate the time for CAS application and visa application process.
It is highly recommended that an International student planning to study in the UK should do so at least before 3 months of the start date of the intake.
Due to the current COVID-19 situation, there may be slight changes in the beginning of the semesters at most universities and it is expected that some universities have and others will extend the date of enrolment at the universities for students arriving late.