Under the Working Time Directive (which applies to most workers and sets out the minimum standards for working hours), your employer has to offer you a break of at least 20 minutes if the working day is longer than 6 hours. There is no requirement under the minimum standards though for your employer to pay you for this break, or to count it towards your working day.
If you’re 16 or 17, you get more generous breaks. You’re entitled to an uninterrupted break of at least thirty minutes when you work for four and a half hours or more.