The only legal entitlement to a break is a single (unpaid) break of 20 minutes if the shift exceeds 6 hours. It's not cumulative, so the fact that your husband works 12 hours wouldn't entitle him to a 40 minute break or to two 20 minute breaks. The entitlement is still to a single 20 minute break. As long as the break occurs part-way through a shift (i.e. not knocked off the beginning or end of the shift) it can be at any time the employer chooses. 'Floating breaks' (i.e. 'when convenient') are lawful as long as the employee gets a solid block of 20 minutes somewhere during the shift. Employees should normally be permitted to leave the premises during their break.
However, certain occupations are exempted from the full provisions of the legislation. One of these is work within the security industry. Security employees may lawfully be denied breaks which most other workers have a legal right to but the employer should put in place arrangements for 'compensatory breaks'. See here:
http://www.direct.gov.uk/en/Employment/Employe es/WorkingHoursAndTimeOff/DG_10029451
then get your husband to phone the ACAS employment helpline (08457 47 47 47) between 8.00am and 6.00pm (Mon-Fri) for specific advice as to how the regulations apply to his job.
Chris