The law states that,

• You must not alter, rearrange or misrepresent the letters or numbers
• Characters must not be moved from one group to the other (e.g. A242 ABC must not be displayed as A242A BC).

Offences may result in any or all of the following:

• A fine of up to £1,000
• The registration mark may be WITHDRAWN
• The vehicle may FAIL the MOT test

Full details of the requirements are included in The Road Vehicles (Display of Registration Marks) Regulations 2001