In the UK, most organisations have an informal and formal process of resolving complaints. An informal process might mean talking to someone about something that you are dissatisfied with or unhappy about. If you have a personal tutor, this is usually the best person to start with. It's advisable to tackle problems as early as possible to avoid them becoming more serious and often the informal process can resolve any issues.
If talking to someone is not an option (perhaps you are unhappy about the person you are supposed to talk to), you may have to start a more formal process by writing to a designated person in your institution. You should be able to find out about the procedures and who to contact by looking on your institution's website or in your course handbook. You can also seek advice and support from your students' union. They will be well-informed and be able to help you understand how the institution's processes work.
There may be time limits for making a complaint. Always keep copies of any documents and correspondence relating to your complaint.
Ideally, complaints will be resolved internally, but if by the end of the process you are not satisfied with the outcome, there are formal organisations in the UK who are set up to deal with individual cases. These are called 'ombudsmans'. See the link for more information as each country in the UK has a different ombudsman. If you are studying for a qualification at a different organisation to where you are studying, see 'Awarding bodies'.