If guidance is not provided, how would one know, how one would comply.
In the broadest sense, I would say by having sufficient knowledge and experience to recognize whether an installation is "reasonably safe" or not. The only official guidelines available, in the form of the Approved Document, don't really impart sufficient knowledge for that. Certainly, they indicate that compliance with BS7671 or one of two dozen other specific standards will be deemed (in the view of the Secretary of State) to satisfy the requirements of Part P, but as those guidelines themselves point out, that doesn't mean that those are the
only methods of compliance. An installation can quite easily be "reasonably safe" without complying in full with any specific published standard.
Reasonable provision shall be made in the design and installation of electrical installations in order to protect persons operating, maintaining, or altering the installations from fire or injury.
Means very little without guidance.
Clearly it's open to some subjectiveness, and my idea of what is "reasonably safe" might differ somewhat from somebody else's idea (especially those brought up in the BS7671 electricians' mindset who often seem to think that anything short of the latest version of BS7671 cannot be considered acceptable and safe).
But even official guidelines are only that - Guidelines. Even the statutory provisions only go as far as suggesting that following the guidelines contained within the approved documents would
tend to establish compliance, and don't actually state that following all the guidelines
will be taken as indicating compliance (which contrasts somewhat with the the comments about "in the view of the Secretary of State" in the approved documents). It's all left somewhat vague.
Should it ever go that far, what is "reasonable" provision for electrical safety would have to come down to the same sort of arguments as to what constitutes reasonable force against an intruder, or what constitutes a reasonable duty of care. As such, it's likely to be open to the same sort of somewhat arbitrary, even capricious, decisions as such matters.