My problem with a blanket approach is that it doesn't allow for or cater for genuine emergencies or specific circumstances that arise like massive demand for work following floods, sayProtection from exploitation needs firm rules which cannot be opted out of.
I'd prefer to give more teeth to existing employment legislation so that employers can't force people to (in this example) work more hours than their individual employment contract allows and to give people where that happens a proper strong recourse - maybe that recourse is already there in a Union ?
Taking into account what I said earlier that I agree there should be limits based around specific jobs where safety & health is an issue, for example medical people, train drivers etc (with those limits set by Heath & Safety people or those that know the industry/sector not government) but not a blanket "one size fits all" limit
As an example, if you have a fixed limit of hours for everyone irrespective of their job type, what happens when you need to call an emergency plumber on a weekend ? If all the plumbers say ... "sorry, can't come out - I've done my maximum hours this week" or it's a company "sorry I can't send anyone, they've all done their maximum hours this week"
Yes I realise that's an extreme example but it should illustrate the point I'm trying to make which is that there has to be some degree of flexibility