Do you get paid for your break? If so then you are technically still working and should be on the premises so that you are available if you should be needed. If you are not being paid than I don't see how your employer can insist. Check your contract though.
The law gives adult workers a minimum 20 minute (unpaid) break, which may be taken away from the workplace. However there are certain jobs where exceptions to the rules on breaks are permitted:
http://www.direct.gov...ndTimeOff/DG_10029451