Just to add, Ive got a number plate with the letters mis-spaced, had it since I got the car (March last year) and so far so good. I have kept the legal typeface and not altered the letters so its still very readable. I think if you play with the letters to make them look like something they are not, or go for mad fonts then you might be more likely to get pulled for them
Trouble is you have to be good, because if you are pulled and you have done nothing wrong then they will find the plates and do you for that.