Guidelines on how "number plates" should be displayed
All number plates including numbers those purchased through's sales scheme must be displayed in accordance with The Road Vehicles (Display of Registration Marks) Regulations 2001 (obtainable from Her Majesty's Stationery Office).
Rules for the display of number plates are set out in law, briefly these are: -
A number plate must be displayed at the front and rear of motor vehicles (with some exceptions).
Number Plates must be easy to read and meet the British Standard.
Lettering should be black on a white plate at the front and a yellow at the rear.
The background surface should be reflex-reflecting but the characters must not.
There are separate requirements for traditional number plates displayed on vehicles constructed before 1 January 1973.
Lettering and spacing must be of a set size. They must conform to one of the groups shown at:- Number plates fitted after 1 September 2001 or Number plates fitted before 1 September 2001.
Number Plate Law
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The registration mark may be WITHDRAWN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
So i suppose the answer is yes, you can fit them upside down as there seems no wording to the contarary