but how would anyone know?
Exactly! How is an mot tester supposed to know when a number plate was made?
As long as you keep to legal font, standard spacing and minimal/standard looking slogans (I just have 84 mini 25 on mine as I get pissed off with people/testers thinking its an 83) no one is ever going to notice the lack of postcode/plate maker.
Now if you do the 90's thing of italic lettering, messing around with the spacing or replicating foreign number plates then your asking for trouble.