Some rights are legally protected. For example, you are entitled to the minimum wage, and to set amounts of holiday. You are entitled to be a member of a trade union and take part in official industrial action. You are also generally protected against being dismissed for taking action over health and safety — for example, reporting a health and safety problem to your manager.
It is automatically unfair if your employer dismisses you for any of these reasons. You can claim unfair dismissal, regardless of how long you have worked for your employer.