Chance of applying as a prospective PhD student in semesters
In the UK, most PhD studentships for home (UK/EU) will be advertised in the autumn, and also those few places that have PhD programms will have closing dates just after Christmas. In most cases this will be to recruit to programs that will start in the autumn of the following year. 

This is not a hard and fast rule, and plenty of people will get positions later in the year, although many of them will still start at the start of the academic year in October. 

This is not to say that no one starts inbetween, its just much less usual, and often suggests an unorthodox funding route, such as self-funding, foreign government fellowship or industrial sponsorship.