This will depend on the intern’s classification.
In our experience, and subject to the terms and conditions of the internship, a UK intern is generally a fixed-term employee. Employers are legally obliged to pay all employees and “workers” (a classification that sits in between employee and self-employed consultant) the National Minimum/Living Wage (together referred to as the “minimum wage”). In addition to the minimum wage, workers and employees have other statutory rights, including the right to paid holiday, certain rest-breaks, statutory sick pay and family leave and pay etc. – and so interns may be entitled to these too.
Calling someone an “unpaid intern”, “intern” or a “volunteer” will not take them out of the scope of the UK's minimum wage provisions if, in reality, they are a worker or an employee.
Some interns will be genuinely free to come and go as they please - genuine volunteers, for instance - meaning they are not workers (or employees) and are not entitled to the minimum wage and have no statutory rights. Similarly, if an internship only involves shadowing, meaning no work is carried out by the intern and they are only observing, the intern will not be a worker or employee and so the employer does not have to pay the minimum wage.
For details on enforcement action for a failure to pay the minimum wage, see UK: My early employees are willing to work for equity only. Is that okay within the UK market?
Learn More: UK Founder Series: Building Your Team