I would say its Reg 4 of the Electricity at Work Reg

Guidance expanded on in the HSRE25 Memorandum of guidance to EAW

Reg 4(3) being the most applicable

Regulation 4 Systems, work activities and protective equipment
(1) All systems shall at all times be of such construction as to prevent, so far as is reasonably practicable, danger.
(2) As may be necessary to prevent danger, all systems shall be maintained so as to prevent, so far as is reasonably practicable, such danger.
(3) Every work activity, including operation, use and maintenance of a system and work near a system, shall be carried out in such a manner as not to give rise, so far as is reasonably practicable, to danger.

You maybe need to learn the skill of interpreting legislation and guidance in its widest meaning. Often regulations might not specifically say something - that doesn't mean it doesn't necessarily NOT apply.

In Reg4(3) 'every work activity' and 'work near a system' could include general people in the area of a distribution board. If there is'danger' you need to prevent access to live conductors.

Then there is of course the standard 'risk assessment' (Reg 3 MHSWR) - which is most likely to conclude locking/preventing access to DBs to non-competent electricians is a good idea.