From the
Government Your number plates should:
be made from a reflective material
display black characters on a white background (front plate)
display black characters on a yellow background (rear plate)
not have a background pattern
Characters on a number plate can be 3D.
There's also a
leaflet , summary:
characters on number plates purchased from
1 September 2001, will need to meet the following
specifications.
n Character height 79mm
n Character width (except the figure 1 or letter I) 50mm
n Character stroke 14mm
n Space between characters 11mm
n Space between groups 33mm
n Top, bottom and side margins (minimum) 11mm
n Vertical space between groups 19mm