Sub39h - sound advice right there.
I'm sure garrardrj will back me on this.
Pressed plates are only an issue when they don't conform to legislation. I speak to, and came across, people who told me "but I bought it from ebay and they said 'cos it has a BS number, it's fine!"
The amount people that go on about "it's got a BS number on"!! errr, so what! Just because it has a BS number on it, does NOT mean it's legal.
If the following are fulfiled you have legal plates. Dubmeister/Europl8 do fully legal ones and paid £££ to get BS approval (which is what gives them the right to stamp it with a BS mark).
UK spec font as set down in legislation
Correct spacing for style of plate (ie: XX52 XXX or XXX 1234.....not X J52ON X etc)
The background is retroflective (if it is and complies with the standards set by the British Standard for numberplate, you can stick the BS number down).
The maker's name and postcode
If you have all the above, you are legal. If you get a ticket, contest it, have your day in court and you WILL win. Simple. I have had it once, unfortunately the CPS decided to drop it 5 day before court when they found out I knew what I was talking about. It has also been mentioned in two other stops. Both said fine in the end.
The DVLA will only find out IF the police officer fills in a DVLA form, takes photos of the plates and gets you to sign the form (or you refuse). DVLA won't take any other way. They then write to you going, sort it out or else. Depending on the severity will determine how quickly they withdraw the plate. Sometimes if it's that bad, DVLA will withdraw it straight away. Others you'll get one/two warnings. Regardless, the DVLA will only do this to cherish transfer plates (ie: private regs).
The problem is pressed plates (ie: metal) are not natually retroflective. They can be treated to be and that is what makes them comply with the British Standard. If you do buy a set of pressed plates, this is key if you get a ticket and want to go to court. If it is not retorflective, game over. Take a photo of the plate at night with a flash. It it reflects, it's retroflective. If it doesn't, it's not.
@Baz300 - the guy is right in a sense as metal itself doesn't reflect, but as said, depending on where you got your plates, they may well actually comply with the BS.