A semester is 15 weeks. An academic year of 30 weeks comprises two semesters. It may also be possible to start at other times of the year - such as in summer. The University or College in the UK would let you know when you can start your course. Nevertheless, if you currently study at a university outside the USA or EU regions, your university may still accept such an arrangement if it makes a valuable contribution to your degree programme.
Tuition fees vary enormously, depending on the UK university or college and the subjects chosen.
Nevertheless, studying in the UK for only one or two semesters really can be affordable. However, students from countries outside the EU may only work in the UK if they are enrolled on a course of six months or longer i.e. two semesters. If you want to work, you must obtain the work yourself once you have commenced the study programme. If your English is not quite up to standard, English classes can be arranged. Many students following a programme over two semesters often choose to study English language during the first semester to bring their English language up to an acceptable level.