Care to explain why?
As far as I can see the only requirement is for the plate to be vertical (or as close as possible) and visible from in front of the vehicle.
The Road Vehicles (Display of Registration Marks) Regulations 2001 applies.
Until a particularly clued up PC tells me to move it that's where it'll stay.
I fitted a square one on the side of my commercial winch bumper, its not a standard size though it was what they called a Jap import size which is smaller than a UK square plate, I'm going to see how it goes and if it breaks I will order a metal square motorbike one off eBay.