Hi m1llymolly, firstly the police are wrong, if you get no satisfaction from the officer on the desk ask to speak to CID or a superior.
You can buy "show plates" which I had to do (made to as near as damn it british standards) as my car is an american import so will not take the full size plate. Legaly we can use 12" x 6" plates with motorcycle size font, but most plate makers will opnly make full sized ones which are useless.
Regarding "cloning" it DOES happen as people avoid speeding fines, etc, and as the law stands if you sell a car and not declare it by sending in the V5 you are still liable for parking fines, road tax, etc. I saw a case on tv once where someone was trying to claim the car on the speed camera photos wasn't theirs and must be a cloned car.. but when the photo was enlarged it showed a GB sticker on the boot and another sticker advertising a car sales garage in the back window.. the chances of someone copying minor details like stickers are remote. I would contact the police station where the speeding ticket was issued and talk to them. Perhaps send a photo of the back of your car to them or ask for a copy of the photo the camera took. It may be possible to claim compensation from someone for the cost of having to buy another plate (or part cost) if you can prove stress and anxiety occured. If you haven't contacted them already, contact DVLA in Swansea.