I think there is a need to stand back a little and review what has been said, and then try to clarify the position as I see it:-
The whole PIR thing comes from the fact that the Act of Parliament concerning electrical power supplies has a clause that effectively says that no one will cause or allow an unsafe electrical system to be connected to the mains supply. (A caravan is a typical example of a system, but it is not limited to just caravans).
Reversing the logic slightly, this means that the owner of a connection point is liable if a faulty system is connected. So the crux of the matter is how the owner of a connection have confidence that a system (e.g. a caravan) is safe?
In the UK the Institute of Electrical Engineers (IEE) is recognised as the body that devises methods of specifying electrical systems. They have over the years reviewed practices for the installation and testing of wiring, and produce the Wiring Regulations. These are designed to aide compliance with the Act or Parliament or the "How to do it" bit.
Whist it is generally accepted that the IEE is the authority in such matters, and installers following the IEE's regulations will normally be sufficient to comply with the Act of Parliament, but if someone else can prove to the satisfaction of the courts that an alternative procedure affords equivalent or better compliance with the Act then that can also stand as an acceptable procedure. Proving such a thing is a bit of an uphill struggle, but it has been done in the past.
As such a "PIR" is not prescribed by the Act of Parliament, but it is the IEE's considered opinion that it is the way to show the condition of a system.
Now putting that into a caravanning context;
There is a considerable difference between touring caravans (inc. motor homes) and static caravans (and Mobile Homes) Touring caravans are designed to be moved from site to site and will need to hook up to many different electrical points. They are subjet to levels of vibration and stress that may well render some aspects of their electrical system unsafe. Here each site owner must comply with the Act of Parliament and will need to prevent unsafe caravans from being connected to their hook up points. How do they do that?
There are three choices:
A) Trust to luck and assume that all caravans are ok, The problem here is that if an unsafe caravan is connected, regardless of whether it is causes a problem or not, if it is detected the site owner is liable under the Act.
B) Do an inspection of each caravan as it arrives. This would be very unpopular and time consuming and cause unreasonable bottle necks when caravans arrive on site. - But it is the most reliable method assuming the site operator has the properly trained staff to carry out the inspectin.
C) Have some scheme where by they can take the evidence of an independent assessor as being a reasonable compromise. The PIR is the attempt to do this.
Mobile homes and static caravans are not considered to be truly mobile. They are installed on a site and in most cases the electrical system is hard wired to the mains supply. This mans the site operator would need to use a qualified electrician to make the connection. It is the role of every electrician to ensure they only connect safe systems to the mains supply, so compliance with the act is achieved.