If you are a resident of the UK, you are not allowed to drive any car that is not registered in the UK with only a few exceptions.I found this
There must be some sign of what country the vehicle is registered in on the rear of the car. This can be added to the car specifically for your trip or be a part of the registration number.
How to drive a foreign car in the UK? If you’re staying in the UK you may want to consider using your own car. Learn about quirks about driving in the UK.katsgoneglobal.com
This, we also bought a small magnetic UK one for a recent road trip through France and Spain and had no problems at all. I don't understand why the smaller one would be illegal - it's a lot bigger than the EU country id on number platesIt was when we were in the EU, but no longer.
Our government has also changed us from GB to UK.
We bought a magnetic one for our holiday in France, and stored it when home.