Yes, I know that would be illegal - it's considered, variously, defacing, counterfeiting, or vandalizing the state-issued license plate.
I know this because on classic Minis, when an US-sized plate is fitted to a pre-Mk.III boot lid, the size of the plate fouls the hinged number plate drop down feature (intended to keep the UK plate visible when the Mini is driven with stuff plied high upon the opened boot lid). A good number of classic Mk. I Mini owners in the US have cut several inches off of their US state rear plates to make them fit the rear number plate bracket and at least one

found out the law don't like it!
