Apologies if I'm wrong on this, but that relates to character spacing more than plate shape. Whilst square plates are permitted (I'm not denying that, btw) I'm sure there is something (which I accept may have assumed to be law) that states something along the lines of (I can't remember the exact wording and I can't be ar*sed to look it up ) that if there's a "suitable space" for a rectangular plate then it must be rectangular. Certain cars - such as Land Rovers, Evos, etc, (with regards to taxi's, it's more specifically the lack of space for the required information to be displayed along with the plate leads to the lack of space) - don't offer a "suitable space" to mount a rectangular plate and so comply; hence my comment regarding the rectangular mount.
Not meaning to be argumentative here or anything, but I'm adamant there is something along those lines in place. If this is the case, and as I tried to make out in my last post, you'd be incredibly unlucky for it ever to be an issue providing you remove the rectangular mount... which would just look stupid with a square plate anyway in fairness.