Plus, it's only a C&U reg. so no points on license, just a 60 quid fine.
If they decide to deal with you through a fixed penalty, which they don't have to. It's at the officer's discretion. If you are pulled for a registration offence more than a couple of times, or are cocky and mouthy, or come up with 'Oh no officer I know it's illegal but they are just show plates and I have the real ones in the boot' (show plates illegal too incidentally) , you are quite likely to end up in court. The maximum penalty is £1000.00, plus withdrawal of the registration and probably something else that I can't remember too. Withdrawal of the registration means getting your car re-registered which is a major pain.