when you say border you just mean distance from letter to plate edge?
So the importany bit really is the border then.
[taken from DVLA web]
Your number plate should show the correct:
•character height - 79 millimetres
•character width - 50 millimetres (except the figure 1 or letter I)
•character stroke - 14 millimetres
•space between characters - 11 millimetres
•space between groups - 33 millimetres
•top, bottom and side margins - minimum 11 millimetres
•space between vertical lines - 19 millimetres