There's the difference between a PPL test and a CPL test.
I realise that in the UK that most CPL pilots go onto bigger stuff comparatively quickly compared to Australia (and the US) but now you're supposed to be a professional and you cannot assume that the passenger (aka Examiner) knows how they work.
You are legally obliged to demonstate these things on a CPL flight. If you don't, you can find yourself in court very quickly.
So, if you want to play safe, ask if the examiner needs to have the seat belts explained etc. You can start the brief etc and if the examiner is happy, s/he'll will say that's ok and for you to continue.