GU10 LED lights strongly strobing (no dimmer) UK
a regular switch (i.e. not a dimmer).


Although dimmers are the main culprit for this sort of problem, there are other devices that can cause similar problems when used on the same circuit. Especially devices that operate in switch-loops without neutral. Perhaps you have a switch-locator light or a smart switch? 


  they work at mains voltage so there shouldn't be any transformer or led driver behind the fitting,


There will be a small driver circuit inside the GU10 lamp housing. Due to size constraints it will be a very simple circuit and cheaper products may be more susceptible to this sort of problem.