The Trade Union Congress (TUC) has a range of information, advice and guidance on wellbeing within the workplace:

The Health and Safety Executive (HSE)  has produced HSE Management Standards - a framework (not itself a legal requirement) which can be used by employers to assist in meeting their legal requirements.

The Management Standards implementation process is supported by a workbook which provides tips, advice and guidance from people who have gone through the process. It includes a selection of checklists to allow employers to be sure that each step has been achieved before they move on.  The checklists can also be used by employees as a benchmark of best practice which should apply in all workplaces to reduce stress.

The Chartered Institute of Personnel and Development, along with mental health charity MIND have published a revised mental health guide for managers to improve support for those experiencing stress and mental health issues at work. 

Mentally Healthy Schools provides information and resources around supporting staff wellbeing.