CoMAH is the UK's national legislation in response to the requirement to implement the Seveso II directive (European directive 2003/105/EC). It replaced the CIMAH legislation which was the UK's national legislation in response to the requirement to implement the Seveso directive (European directive 96/82/EC). It only applies to 'high hazard' plants (as defined in the directive, and relates to quantities of toxic, flammable and named substances held on site).
All workplaces in the UK are covered by 'The management of health and safety at work regulations'(1992 and updated in 1999). This requires all businesses to carry out risk assessments of all their activities to employees and others, and is the UK implementation of European directive 89/391/EEC.
In addition, workplaces in the UK are covered by the 1974 Health and Safety at Work act(HSAWA), which requires companies to prove that they have ensured the health and safety of employees as far as reasonably practicable. Note that the burden of proof rests with the company, not with the Government. To offset this, the proof is on the basis of balance of probabilities rather than beyond reasonable doubt. Nevertheless, this is arguably still the most onerous and all encompassing piece of health and safety legislation ever introduced into the UK, and possibly the world. Interestingly, it also places some duties on employees. In theory, it is possible for an employee to be prosecuted for causing an injury he or she sustained at work, although in practice such prosecutions are exceedingly rare. Almost all prosecutions in the UK relating to health and safety at work are carried out invoking the HSAWA, although it may be in conjunction with other legislation as well.
None of these specifically state a HAZOP or MOC as a requirement. All, however, require, or imply, an assessment of the risk of a business' operation on employees. If a HAZOP is not carried out on a process plant, an alternative adequate risk assessment is required. Often, in practice, this implies justifying why a HAZOP is not required. However, for particularly high hazard processes, a HAZOP may be deemed to be insufficient.