I got pulled in Manchester by the Police and Vosa doing stop checks - they did point out to me that my pressed plates were obviously ilegal, which I acknowlegded - they were cool and didn't fine me or anything, just advised me to swap them back to the legit ones when I get 5minutes! However, this was a cool cop and I noticed someone else at the same area (loads of cars being pulled by loads of cops!) getting a sixty quid fine!
My advise is, if you really want to run illegal plates, keep some legit ones in your boot.