And that's what I'd expect, on smaller cars they look normal and don't immediately stand out. But there is no getting away from the fact it is illegal and you've got no excuse to get out of a ticket if you get a BIB having a bad day. Their argument will be the smaller characters are less able to be read by ANPR. The other thing I've heard quoted on other forums, is plates are supposed to have the details of who made the plate stamped on it somewhere, something you won't get with a non road legal plate. It's just something else the BIB can use to argue against any "I had no idea it was illegal" claims.
10+ years ago I got a ticket through the post as I was caught on an ANPR camera for having a non conforming plate (3/4 plate on my Elise) I was so mad, it had obviously done it's job as they'd identified me, so what was the problem? The notification asked for the name of the driver. So with confirmation that I was driving, I sent a covering letter stating that I was 250 miles away from home, didn't have my log book with me and my standard plate had come off when I hit some debris on the A1. Rather than run miles back down the busy A1 looking for it in the middle of a busy carriageway, I put on my "show plate", better than no plate I thought and would do me until I got home. I then asked, if you're still going to fine me, please advise what you would I should do in that scenario in the future? Surely 3/4 plate is better than no plate? It did it's job? Alternative, sit on the hard shoulder of the A1 waiting to get recovered? Anyway a week or so later the fine come through with no response to my letter. From that day onwards I've never run a plate on my fun cars.