'No supporter enters the field of play.' or 'No persons are permitted to encroach on playing area'?At this point I'm out.
It's not up to the law to define exactly what a business or operation (such as a football club) does to protect its staff, customers and the public, because the law does not know the ins and outs of every business, let alone the layout of every commercial premises and football stadium in the land so what you are asking for is ludicrous...a football club should be it's own expert on how it protects its staff, same as a factory should consult the right people and be it's own expert in protecting its staff.
The legislation is abundantly clear. No supporter enters the field of play.
In some fantasy land the law might have the time to analyse the security and health and safety arrangements of every premises and piece of commercial property in the land providing bespoke guidance in every case.
Here in reality there is a catch all, very clear instruction that it is the duty of the occupier of a business to take reasonable steps (the word reasonable used because the law cannot know everything about every business) to protect staff/customers/the public.
If anything the injured players in pitch invasions have every right to pursue a case in tort law against the club for negligence.
There is a difference in these concepts. And this rule still applies to the persons who enter the field.
And the rule regarding the club should sound more specific, for example: The club must fence off the field from the possibility of exit of fans with a fence 3-5 meters high.
If not fenced, then the club broke the rule, if fenced, but someone did run out - what did the club break?
Otherwise, it turns out that if someone needs to get the club fined, he can just run out onto the field! But what if the competitors need it? Or maybe the association itself needs it?