Hmm might not be, after all they're not used anymore.. 
mmmyeah, I'm not really sure what you could do with it. An ANPR camera would check your number plate against the electronic database for valid tax. You would either have tax or not, it would not matter about a bit of paper in the windscreen.
Maybe it could have been a legal problem a few years ago, but not now.
Replica tax discs seem quite popular now the law regarding tax disc display has changed:
http://www.creativetaxdiscs.co.uk/I might get a '68 one for the bus
