I did read somewhere that if the front numberplate is retained then it must be used for its original purpose, ie to carry the reg no. If it has other stuff on it - logos, manufactuere decals etc or even left blank then it becomes classed as an "ornament" and could be considered to be dangerous to pedestrians and therefore illegal.
Having said that, on my Sunbeam S7D I have a front plate fitted with a large Sunbeam decal

Jim