I have been stopped 3 times (in three weeks) recently for having "smaller letters/numbers" on the back of my JDM... this is because I wanted my private pate to read in one line so reduced the size of the figures (maybe got a little carried away with it!)
Apparently they have had to crack down of "illegal" number plates because traffic cameras don't pick them up.
The last time I was stopped I was doing 65mph on the motorway (for once) and was overtaken by a merc doing about 90mph, but obviously posed a greater or easier target/risk to the patrolman who was just pulling off the hard shoulder at the time. Not that I'm bitter about having a 10 minute lecture on number plates and traffic camera's!
I have changed the plates back to normal now as its too much hassle....