Thank you for that ref, PseudoBadger.
Health and Safety at Work etc. Act 1974:
"Section 3 General duties of employers and self-employed to persons other than their employees.
"(1)It shall be the duty of every employer to conduct his undertaking in such a way as to ensure, so far as is reasonably practicable, that persons not in his employment who may be affected thereby are not thereby exposed to risks to their health or safety."
Which could possibly cover a multitude.
For employees, however, specific duties are spelled out in section 2, eg
"Section 2 (2)(c) the provision of such information, instruction, training and supervision as is necessary to ensure, so far as is reasonably practicable, the health and safety at work of his employees"
But iiuc, workfarers are probably not covered by Employment legislation - which specifies things like hours worked, toilet and meal breaks, etc. In fact, I've seen complaints about toilet breaks and working conditions in previous workfare schemes. These will impact all workfarers but may have particular impacts on those with health problems.