What makes a Legal Number Plate?

Font

• All number plates made after 31st August 2001 must display the mandatory font.
• Stylised letters or fixing bolts to alter the appearance of the letters are not legal.

The law states

• You must not alter, rearrange or misrepresent the letters or numbers
• You must not move characters from one group to the other For Example: A123 ABC must not be displayed as A123A BC
• No stylised letters
• Don't misrepresent letters / numbers using screws or fixings
• Don't misrepresent letters / numbers using squashed or distorted letters
• No images other than the approved flags

The British Standard for number plates?

The British Standard sets out the physical characteristics of the number plate. This includes visibility, strength and reflectivity.

The British Standard also requires each number plate to be permanently and legibly marked with the following information:

• The British Standard number (currently BS AU 145d)
• The name, trademark or other means of identification of the manufacturer or component supplier (The company that made the plate)
• Name and postcode of the supplying outlet.

Number plate options

• You may add a non-reflective border and/or Euro-symbol with national identification letters

This information has been gathered from the DVLA website. 
More information concerning what makes a legal number plate can be found there.