The simple answer to your question has to be 'No'.
Under health & safety legislation all employers are required to carry out risk assessments for the duties carried out by their staff, and then take action to eliminate (or to minimise as far as is reasonably possible) any risks which are identified. So an employer is required to take reasonable steps to ensure his staff's safety but there are no automatic rules about locking up pubs late at night.
(When I worked on the railways, I was employed at a fairly large railway station, very close to a busy nightclub. Our staff frequently had to deal with large groups of drunks but there was no requirement for a male member of staff to be present. The night shift was frequently comprised entirely of just two women, who would have been seriously offended if you suggested that they needed a fella' just to sort out a few p!ss artists!).
Chris