CDM, COSHH, welfare regs, they are all children of one parent: the Health and Safety at Work etc. Act 1974. It is the Act inspectors prosecute under, and its logic explains why welfare provision is nobody’s optional extra.
The two duties that matter here
Section 2 puts a general duty on every employer to ensure, so far as is reasonably practicable, the health, safety and welfare at work of their employees. Welfare is in the wording of the duty itself, not an add-on. That is the root of the requirement for toilets, washing, rest and warmth wherever people work, including a muddy field.
Section 3 extends the duty beyond your own payroll: employers and the self-employed must not expose other people to risks from their work. On a multi-contractor site, that is why the principal contractor’s welfare provision covers subcontractors’ operatives, and why "they are not my employees" has never been a defence.
Reasonably practicable, honestly
The Act’s famous qualifier balances risk against the cost, time and trouble of controlling it. Two things follow. First, the balance tilts with severity: basic sanitation and warmth are cheap to provide and serious to omit, so the law expects them provided in almost every circumstance. Second, once a risk materialises, the burden of proving something was not reasonably practicable sits with the duty holder, in court, after the event. A weekly welfare hire is a very small number against that.
Who enforces it
The HSE (and local authorities for some premises) inspect, issue improvement and prohibition notices, and prosecute. Welfare failures feature regularly in notices on small sites because they are visible within minutes of an inspector arriving. The specific construction requirements sit in CDM 2015, our CDM guide covers Schedule 2, but the power behind them is this Act.
Plain-English summary, not legal advice. The Act is at legislation.gov.uk and HSE guidance at hse.gov.uk.