how olds your bike... if its after 2001 you are under arrest sunshine
Guidelines for the display of number plates for vehicles registered from March 21st 2001 are detailed under British Standard BS AU 145d. As per these guidelines number plates must be made of reflective material (not retroactively made reflective by treatment). Aluminium, which traditionally makes up pressed number plates, does not meet this standard. In 2021, this changed to BS AU145e, with a more robust material introduced.
Another standard requires the material to be flexible, and must be able to spring back to its original shape if bent. Metal plates cannot do this.
These two requirements means that pressed metal number plates are effectively illegal for any modern vehicle