Apparently the reason is they are not made of a retroreflective material and also because the letters are not flat to the plate and cannot be read by ANPR.
Not true. My pressed plates are reflective, they're almost shinier than the pure white car itself 
I've never had an issue in the past either with electronic toll booths being able to read it and (very unfortunately for me!) the coppers had no trouble reading it with their ANPR cameras either when they nabbed me for speeding.
I've read into this quite a bit, and from what I can gather, pressed metal plates are not legal in the UK. As already pointed out, the specification states that they must be made from a retroreflective material (which is not steel or aluminium) There is distinct difference between reflective and retroreflective. A reflector scatters light a bit, and reflects it back at the same angle of incidence, meaning unless the light beam is 90 degrees to the surface, it will be reflected away from the source. A retroreflector minimises scattering, and reflects light back towards the source somewhat regardless of the angle of incidence. Raised letters also cause issues with scattering.
That's not to say a pressed metal place cannot be read by certain things, but by definition, they are illegal in the UK when fitted to cars registered on or after 1st Sepetmeber 2001.
