Easiest and most relevant is the Health and Safety at work etc. Act 1974

section 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;

It is also referenced in the managment of health and safety regulations, in regards to working with young people (19 3 (b))

also management regs.

Health and safety arrangements

5.—(1) Every employer shall make and give effect to such arrangements as are appropriate, having regard to the nature of his activities and the size of his undertaking, for the effective planning, organisation, control, monitoring and review of the preventive and protective measures.

(2) Where the employer employs five or more employees, he shall record the arrangements referred to in paragraph (1).

While it does not use the word supervise i wonder how you can do any of those things without surevision being part of it. The ACOP that went with them used to make it very clear - but unfortunatly it was witdrawn under the drive to stop "gold plateing" safety

There  was a case some years back  when Westminster Council (I think) prosecuted a lift maintenance company after one of their engineers fell down a lift shaft. One of the things they did them for was a “lack of supervision” but it was never explained how an employer is meant to supervise peripatetic staff unless they assign a manager to follow them around, which is of course ridiculous.