Sorry c20ona but, you mates aren't exactly accurate. There are specific guidelines to what is and isn't legal when it comes to numberplate size, layout, spacing and digit size etc.. and it applies to both the front and back plates. See the DVLA website for details. However, with imports we are permitted to what amounts to motorbike size plates etc...
On mine I have had a plate made on the front that just fits the front no plate mount with no overhang so, no decreased air flow. I only have 5 digits on mine though ! This was made by, cough, Carnoiseur.
I wouldn't worry too much about the Polices view though as they very rarely know the full details as to what is and isn't legal with no plates. When it comes to things like that, they are usually clueless, unless they're jobs worths !
HTH