The square number plate that has been receiving a lot of comments is, I believe, motorcycle-size plate. They are allowed to use smaller digits than used on cars etc. due to the limited space.
I had a small oblong plate with m/cycle digits on the front of a Caterham a few years ago because a standard plate looked over-powering on a car with such a small frontal area.
I guess they are not strictly legal, but are unlikely to draw any attention as they appear to be the right dimensions on a small car like the Up.