Badgerchops is quite right, except for the source of the rules on breaks for computer users. Not the infamous Working Time Regulations, but The Health and Safety (Display Screen Equipment) Regulations 1992, which state in section 4:
�Daily work routine of users
Every employer shall so plan the activities of users at work in his undertaking that their daily work on display screen equipment is periodically interrupted by such breaks or changes of activity as reduce their workload at that equipment.�
http://www.opsi.gov.uk/si/si1992/Uksi_19922792 _en_1.htm
I�m mentioning this in case you choose to quote the legislative source to your employer when you tackle them.
You are entitled to breaks, but rather than just take them and open yourself up to a disciplinary for insubordination, ask for a quick meeting with your manager and explain what you�ve discovered diplomatically. If they get stroppy, raise a formal grievance but use the procedure rather than go AWOL. It�ll protect you later if you want to make a claim for unfair dismissal.