Failing to display carries a £1000 fine, but as GC pointed out the revenue boys only sit in their vans pointing numberplate cameras at everyone these days. I know a guy who was done, but it was only part of a list. Doing 65 in a 30 zone with a loud pipe, small numberplate and the tax disk in an envelope at home wasn't Chris' best day, although with 9 points already you can sort of see it was going to happen in the end. He kept his license, the police must have been fairly gutted, but insurance on a Ducati is now rather hard to find.
You're still failing to display, but putting a colour photo copy in the tax disk holder and keeping the original on you is perhaps a useful bit of camouflage that could theoretically (and in no way encouraging you to break the law) survive fairly close scrutiny. IMHO, write copy across it rather plain ink to avoid any charges of falsifying documents.
Andy