It's a grey area this one, as imported cars are allowed to use motorcycle numbers on two lines, because they don't normally have a mounting area that can take a one-line plate...
If you make sure it's spaced correctly and is legible, it's unlikely you'll have an issue - strictly speaking, my front plate should be on two lines as it's using motorcycle lettering, but I passed MOT no problems - but then again, mine's an import, so they might just have been more lenient...
:)