I have always had a set of 'legal' plates in the boot when I have had slightly 're-arranged' number plates. Never needed any of them (yet).Always have the yellow/white plates in the boot. If you get stopped ( unlikely) then it's "Sorry officer, I was at a car show and I forgot to change my plates back" Hopefully a wrapped knuckle and on your way. They're probably more interested in looking at your stunning car than doing you for such a minor issue.