summarised the gov guidelines
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.
taken from
http://www.regtransfers.co.uk/main/car_numberplates_regulations.aspdepending on the officer as far as i know you either get a Vehicle defect rectification notice, which gives you 7 days to remove the "defect" (mod) get the ticket stamped to say the car is legal and take into a police station,
or
you get a ticket. again depending on what the mod is depends what fine you get.
