the demo would probably be white but would be marked as a demo in some way, sticker, stamp, "x" on the side they are promoting etc. I would assume they would avoid tax on demos.....not being an expert on customs and excise I'm not really sure, some one else might be able to give a more definitive answer.
The record still commanding a price of around £1000 but think thats way too much IMHO considering the amount for sale in recent times.