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
That sums it up nicely IMHO- I had to change mine to comply when a nice policeman asked me to last year, Over the years I've been stopped four times and the last time it was serious - 'get it changed or else' - I guess it's to do with the number plate recognition cameras that are prevalent now.
The only thing you haven't mentioned and what I'm still not sure about, if your number is short, say A1 ZED, is it allowed to cut down the plate?