Under the Working Time Directive (which applies to most adult workers and sets out the minimum standards for working hours), your employer has to offer a break of at least 20 minutes if the working day is longer than six hours. Under these minimum standards, if your lunch break is over 20 minutes, it counts as your full entitlement to a break for that day.
Where your work is monotonous or the work-rate is predetermined, you have a separate right to adequate rest breaks (Regulation 8 of the Working Time Regulations). This is on top of the basic 20-minute break.
There are also separate rules about rest breaks where you operate a computer at work, found in the Health and Safety (Display Screen Equipment) Regulations 1992. In particular, regular breaks away from equipment are essential to combat the risk of repetitive strain injury.
Any additional breaks could well depend on the terms of your contract, so check your staff handbook for more information. Just as most people are paid more than the minimum wage, most employers offer more than the absolute minimum on breaks as well.
This will probably be the case if you've got a recognised trade union where you work (as breaks and hours are a priority for unions), so talk to your workplace rep if you have one.