There are no 'fees' as such. Payment is made by purchasing the course materials. In addition there is a fee for registering for the examination for each course. Each course has a unique serial number, and only one student may sit the examination for each serial number. It is not necessary to buy all the courses at once, but buy as you study.
Most students buy one or two at a time, and complete four courses a year.
However, some buy less and some buy more. It is entirely up to you. UK universities are not accredited in the same way that American universities are. This is because UK universities are set up and monitored by the UK Government and have to adhere to rigorous standards. Examination scripts are marked by internal lecturers and then a sample, plus all marginal papers are also reviewed by external exmainers drawn from other leading Universities.