I had a private plate with dodgy lettering for about 5 years on my M3 and never had any trouble. I once had a policeman shout out of the window of his car to me at a roundabout to get the plates changed, but I was never officially pulled over about it.
Of course, it caused me an MOT failure, but the following years I remembered to swap them the night before for regular ones, and just put the dodgy ones back on the next day!
I'm currently waiting for the DVLA paperwork to put the same dodgy plate on my Z, so will wait and see if having the plate on an attention-attracting car will cause more trouble :?
Jody