I have posted up the regulations in the past (after having them handed to me ) however DVLA don't make it easy to find regulations online.
Here they are in brief though:
SUMMARY: What is required and permitted on UK road-legal number plates
Required:
* White front plate (to British Standard BS AU 145d)
* Yellow rear plate (to British Standard BS AU 145d)
* The registration number of the bearer vehicle in the mandatory font (black ‘Charles Wright 2001’)
* Spacing of characters and character groups in accordance with the measurements specified in the regulations. Variation is not permitted
Permitted but not required:
* 3D variation of the mandatory ‘Charles Wright 2001’ font
* Coloured, non-reflective border
* National emblem: English St George Cross with “ENG†legend, Scottish St Andrew Cross with “SCO†legend, Welsh Dragon with “WALES†and “CYMRU†legend, British Union Flag with “GB†legend) or Euro Stars symbol with “GB†legend.
No additions or variations are permitted. Common illegal variations are:
* Fancy or decorative typefaces
* Bolts placed in such a way as to alter the appearance of characters
* Adjusted spacing between characters or character groups
* Altered characters
* Additional logos or symbols, such as sporting emblems and religious symbols.
There are concessions for older and vintage cars. “Historic vehiclesâ€, i.e. those built prior to 1973, are permitted to bear the old-style black plates of either plastic or traditional metal construction.
To quote the DVLA: "Vehicles constructed before 1.1.73 may display traditional style ‘black and white’ plates i.e. white, silver or grey characters on a black plate."