No comfort to you, I know, but the reason you were done is because there's a big purge on 'illegal' numberplates at the moment. This is tied in with the ever-wider introduction of ANPR - Automatic Number Plate Recognition - which instantly reads a number plate, checks whether the vehicle is stolen or otherwise wanted in connection with crime, and is taxed, and insured. It is having a lot of major successes in many areas of policing.
In particular, a high proportion of serious motoring offences (and collisions) are committed by the untaxed and uninsured so ANPR is helping to get them off the road.
For this to work, of course, requires all numberplates to be clearly visible and the present 'no tolerance' approach is intended to achieve that.
Had he been a real jobsworth he would almost certainly have given you a handful of penalty notices, for obstructed lights, mirrors, etc.
