In that case, adhere to this bit:
Over 2 lines, the minimum height of a UK square plate is 191mm:
11mm margin + 79mm letter height + 11mm space between characters of top and bottom row + 79mm letter height + 11mm margin.
For a 5 digit plate e.g. "A2 AAA", the minimum width of a UK oblong plate is 338mm:
11mm margin + 50mm character width + 11mm space between characters + 50mm character width + 33mm space between groups + 50mm character width + + 11mm space between characters + 50mm character width + 11mm space between characters + 50mm character width + 11mm margin.
So yeah, UK square minimum height = 191mm and UK oblong minimum width = 338mm.
*UNLESS YOUR NUMBER IS '1', IN WHICH CASE:
11mm margin + 50mm character stroke + 11mm space + 14mm character stroke + 33mm space + 50mm character stroke + 11mm space + 50mm character stroke + 11mm space + 50mm character stroke + 11mm margin
A total of 302mm.
That's what I'll be doing with my front plate. That means that you have UK size lettering so police can't give you any trouble whilst the plate is cut right down.