I think that should be fine though. You've got a bigger space between the 3 and the X than the other letters (so the spacing is ok), and there's no rule as far as I know about how long a number plate must be, it's fine to shorten it if you want, as long as you're using the correctly-sized lettering/font.
The spacing is fine, but the lexus logo and the border round the plate are illegal.
The only logo you can have is the GB/Euro and you cant have a border, and you must have the plate suppliers name and no. on it.
I got fined for the same thing, my lettering was standard, but had moved the X over on my L3X plate, i also had the logo and border like Tom has.