The garage wouldnt care for a service, not to sure they would care on a mot, butb they definetly arent going to give a toss about that on a service...
The service just looks after your car mechanically, they then tell you the problems that need sorting but dont have to do it... number plates are low on there priority i would say.
If they are illegal then its the coppers that are going to have you anyway, at the end of the day it depends what were talking about - some of the plates with fancy fonts and stuff look cack anyway and the police spot them a mile off, but dodgy spacing and that doesnt seem to bother them that much anymore...
Number plates are to identify the car so as long as they are legible and clear i dont think your going to have to much hassle.
If they are italic and squirly chuck them out and get
http://www.GTgrafix.comwww.GTgrafix.com to sort you some decent legal ones so you dont look like a tool....