Actually under H&S but more so 'duty of care' it is your problem.
It is possible you could be approached after an accident to give evidence and part of that questioning could be 'why did you ignore it'.
That is very true, and for heath and safety unlike most contracts is must be in writing, although it says nothing about being on paper, so be it an email, text message, or letter you must tell them in writing.
It can simply be, as required under HSE regulations the letter is to confirm my opinion that the lack of RCD etc, earth sizing etc etc, mean this property is not in a safe condition to use as accommodation, i.e. potentially dangerous. Then you are in the clear, even if they through it in the bin, that's not your problem.
However the other issue is if the faults you have found in the eyes of a court would make the premiss potentially dangerous? This is not so easy to prove, we can very easily say it does not comply with the current BS 7671 as a new installation, but every edition of BS 7671 has the date after which designed must apply. So if in 1965 the house wiring was designed without any earths to the lights, (assuming no wall lights) and the design has not altered over the years, then it still complies with the version current at the time of the design.
Well not quite, as before 1992 it was not BS 7671, but some how you would need to show the design has altered, clearly the design has altered in 1965 there were no LED lights, and we find phrases like
Lighting fittings using filament lamps installed in a room having a non-conducting floor, mounted at such a height that they cannot readily be touched and are out of reach of earthed metal.
so installing LED lighting means you need an earth. There are other changes in 2008 RCD's were required for most circuits, and
Where the location containing a bath or shower is in a building with a protective equipotential bonding system in accordance with Regulation 411.3.1.2, supplementary equipotential bonding may be omitted where all of the following conditions are met:
is normally where lack of RCD protection becomes a problem, and we can say lack of the RCD is potentially dangerous.
We have other items like the manufacturers of showers, immersion heaters, and boiler stipulating RCD protection is required. But unless you know what the manufacturers instructions say, it is hard to say lack of RCD is potentially dangerous.
Don't misunderstand what I am saying, I feel there should be RCD protection,
this was in the electrical safety councils best practice guide well past 2008 when RCD protection was required for sockets.
So maybe best is still to do nothing, which would include raising the issue on any forum.