The law says that all employers must safeguard the health and safety of all people affected by their undertaking and, in the UK, it is a criminal offence to fail in this duty of care.
Fundamentally, employers are required to assess the risks associated with their business and manage those risks at a tolerably low level.
More specifically, employers are required to comply with applicable regulations; often a large number of them.
For fixed wiring, the specific regulations are the Electricity at Work Regulations, 1989 (EAW). The HSE has issued a very useful document, which is freely available to download from their website, entitled ‘Memorandum of Guidance on the Electricity at Work Regulations, 1989.’
In spelling-out the requirements of the EAW regulations, the HSE guidance makes it clear that fixed wiring shall be maintained so as to prevent danger (so far as is reasonably practicable) and that regular in-service inspection is an ‘essential’ part of preventative maintenance.
For prescription, you can turn to the British Standard mentioned above (BS 7671: 2008).
Remember: This British Standard is comprised of non-statutory regulations.