Simon
In my neck of the woods, if you carried with you some images of your car on the 2003 Goodwood sculpture, as well as a couple of pics of the original Le Mans No.5, the police would just be dead impressed, wave you on your way or perhaps ask for a look at the engine out of curiosity. I'm not sure if that's the same view the Met would take in your area (as they've today fined me £30 for the spacings on my license plates ... have they really nothing better to do?).
If you don't wish to move house out to the sticks, then pleading ignorance works to a point, but you can only get away with that for so long - are you allowed the white roundels without the numbers? Or perhaps you could change the "5" to a sylised "S", with the plea "It's not a number officer, it's a letter" ...?
Cheers
Rob