Most of the international students prefer September intake for their studies in UK, although January and May intakes are equally good for certain programs.
Why September intake is a major one because:
All courses in the undergraduate and postgraduate levels are available for students.
All universities participate in the admission process.
Russel Group universities are available only for September intake.
For students pursuing final year graduation, their mark sheets and certificates would be ready by June / July only. In that case they choose for September intake.
If desired undergraduate courses are not available for January or May intake.