Agree with the advice given by the others on here. In addition to the police car ANPR cameras there are fixed ones too so no doubt they have already ‘pinged’ some of these. Anyone can report someone using a car without an MOT anonymously. Just google ‘no MOT reporting’ and you can see the procedure on the Gov website. If they are involved in an accident caused by faults that would have been picked up by the annual MOT the consequences could be much more serious than just a fine and points. Maybe worth while printing off this thread and let them read it to see what is at stake if a verbal reminder hasn’t worked.