Very true!, don’t know if it’s a actual requirement but I have heard a tester can refuse to test if the car is either obviously unroadworthy or to caked in muck to see what he’s testing. Definitely applies in commercial setting with HGVs etc.
I personally never like sending in a dirty car for test, there also more likely to pass and just let you know if it has any bits need doing instead of instant fail,
Tester once said to me if a car comes in clean, looked after with say new brakes and it’s obvious owner tries to look after it but say track rod end is shot, tester would still pass but say “iv passed it but does need off side track rod end” and owner would do it, he then said if something comes in filthy, not cared for and generally a reck then he would give as many advisory’s as needed or fail knowing owner wouldn’t spend on car even if it passed and was told it needed few bits.