Years ago I found one of my number plates was missing. I phoned the police, who weren't very interested on the grounds that number plates aren't expensive, so it was hardly a major theft. I pointed out that I'd reported it in case someone had decided to do a ram-raid on a shop using my number plate!
I then phoned the Nissan dealer to get a new plate, and they very helpfully told me that they themselves used an independent garage that was more local to me than the dealership was, and that that garage would be quite a bit cheaper than them - and indeed, it was. I think it cost me a fiver, but the thing that shocked me is that I didn't have to show the garage any form of ID whatsoever, or any proof that I owned the car with that number plate; they made the plate up while I waited, and I gave them a fiver in cash. I don't know if the law has changed since then, but it seemed a pretty easy scam to get away with all sorts of things - parking, speeding, congestion charge cameras and so on, not to mention robberies. All you'd need to do is look for a car (on the street or in a car park) the same make and colour as yours, and order a set of plates with their number.