Curious
Are the company and client policies "necessary"?
If CDM applies then "site rules" is a defined term....
“site rules” means rules which are drawn up for a particular construction site and are necessary for health or safety purposes;
Plenty of contractors and their clients adopt blanket rules which are sometimes far beyond the "necessary".
Is there scope for both your company and your client to review whether the work actually requires safety boots?
....which will require assessment of what those boots are intended to protect against - are you looking at e.g.
(a) dropped objects - so are effective measure in place to stop this happening rather than rely on Personal Proective Equipment (PPE)?
(b) penetrating objects - so are the measures in place to ensure that nails etc are NOT sticking out of the ground, rather than rely on.....?
(c) ankle protection - so is the ground firm and stable, and people are unlikely to trip, rather than rely on....?
I am not going to suggest that it is easy to get a client to flex on their "golden rules" but some are ill thought out.
So, as example, there could be a rule that everyone has to wear a helmet anywhere on site. So, instead of telling the roofer that once there is nothing above them to fall they can take off their helmet, the blanket rule is applied.
Some will then point out that the roofer could fall and bang their head but their helmet is unlikely to be designed to protect against such an incident. In general a helmet (except for the rare ones that provide lateral protection) is designed to protect against falling objects - despite research indicating that well over half of head injuries in construction arise from other sources of danger!
Once you establish that the site rule is in fact necessary, then there is no legal opt out as your worker cannot contract out of the criminal legal responsibilities placed on them and others.