Well no if you'll take the time to absorb what was written, it appears the licence plate cover you have pointed out falls within the legal guidelines ie: NOT illegal.
Must be clear, untinted and uncoloured, iii. Must have surfaces which are flat both on the side which faces the number plate and on the side which faces away from the vehicle, iv. Must have non-reflective characteristics and, v. Must have no unusual refractive characteristics. These requirements effectively prohibit number plate covers which have curved, domed, tinted or reflective surfaces from being fitted to a vehicle. Number plate covers which have lines or other markings on the surface which go over the number plate are also effectively prohibited. Regulation 222(2)(b) of the Road Safety (Vehicles) Regulations 1988 also requires that a number plate fitted to a vehicle must be clearly distinguishable and not be wholly or partially obscured
Looks flat and clear, no distortion and I can't see any lines on it.
You were able to take a clear picture of it weren't you? So...