Contrary to the experience of the earlier poster, mine has been that a common reason for an aircraft in the UK remaining on the N register was reduction of cost and avoidance of the maintenance rules which h the rest of us have to follow. In fact one owner of an N registered aeroplane openly said as much - and he was the MD of a small air charter company (though its aircraft were properly UK and maintained) Eventually he landed his machine wheels up due to a failure.
Well done for bringing the matter to the attention of your MP; I would suggest a gentle nudge to the CAA as well. Far from 'grassing up', the lax approach to N registration encourages behaviours which reflect poorly on all owners and pilots in the minds of the general public. Arguably we all may suffer - the odds are depressingly good that rather than deal with the root problem 'the authorities' would prefer to use an easy popular option even if costs us all.