Short answer:
You are quite correct.
Longer answer:
Building regulations apply where you are building a new property, or extending an existing property, or doing certain types of electrical alterations, (e.g. replacing a consumer unit, adding new circuits), etc.
They do NOT apply when selling an existing property..
They do NOT apply if you are not building a new property..
They do NOT apply if you are not extending an existing property..
They do NOT apply if you are not doing specific electrical alterations..
i.e. you have to get building regulations compliance approval for various types of works..
You do NOT have to get any building regulations compliance approval when you are buying or selling a property..
Therefore whatever building regulations may or may not recommend is irrelevant in your situation.
Otherwise you would also have to comply with all of the other parts of the building regs, not just part P, e.g. Part-M, ensuring all ground floor rooms are wheel-chair accessible, and sockets and switches are raised up / lowered down the wall to make them wheelchair accessible, and that there is a downstairs loo large enough to turn a wheelchair in etc.. etc..!
A periodic inspection to BS7671 carried out by a competent person, (no need to be NICEIC), would result in an Electrical Installation Condition Report being issued, giving an overall evaluation if the installation is 'satisfactory' or 'unsatisfactory' for continued use, with any observations/anomalies coded..
An industry recognised guide for coding observations on EICR's is best practice guide 4.
Where the third item up from the bottom of page 20 suggests a plastic CU, located under wooden stairs or the sole escape route is just a code "C3" improvement recommended..
Which is NOT an immediate or potential danger and would not prevent an installation from being considered satisfactory.. Free PDF copy a available:-
https://www.electricalsafetyfirst.org.uk/media/fpton1au/bpg4-a5-2022.pdf
In summary the bloke is talking Tosh!!