UBU's leadership are democratically elected by the membership — i.e. the students. And crucially, they all come from within that body of students.
The main elections, for the Sabbatical Officers and most of the rest of UBU Council (such as the Non-sabbatical Officers and Lay Members), typically take place from February to March. It's a lengthy process because there is a period of open nominations, then a period of briefings, hustings, campaigning, and finally a period of voting. The dates move because of Easter and major sports events, and we try to avoid clashes.
The people elected in those springtime elections will serve during the following academic year.
A few remaining posts are filled in a further round of elections in October. These typically include any posts that could not be filled in the spring, but they also include the Postgraduate Officer (to give an opportunity for all the many postgrads who are only here for one year).