Where does it say you can go and use a handheld GPS to fly an instrument approach?
Where does it say you can't?
Of course only an idiot would fly an ILS using a GPS...
To fly a nonprecision approach (VOR or NDB) using a GPS, one really needs one with OBS mode and few handhelds have got that. Most modern pilots fly them with an IFR GPS's OBS mode.
Where would you stand at the board of enquiry when a person you have taught has crashed their plane because you didn't teach them the pitfalls of GPS and the need to have equipment approved and certified before use?
Nowhere. It would not arise. In the UK, there is no comeback on an instructor. If there was, many would be out of a job, following accidents of former students.