Check out my new ass

I went ahead and did some digging, as i knew your location would fall foul if a VOSA lad found it but i wanted to actually know why.

Subsection of The Road Vehicles (Display of Registration Marks) Regulations 2001

Council Directive 93/94/EEC

Relating to the space for the mounting the rear registration plate of two or three-wheel vehicles.

Section 2 - General Location

2.1.1 - The plate can be positioned within the longitudinal planes passing through the outer extremities of the vehicle.

Section 3 - Inclination

3.1.1 - Must be right angles to the median longitudinal plane of the vehicle;

3.1.2 - May be inclined from the vertical by no more 30° when the backing plate for the registration number faces upwards;

3.1.3 - May be inclined by not more than 15° from the vertical when the backing plate for the registration faces downwards.

6 - Geometric Visibility

6.1 - The space for the mounting the plate must be visible within a space bordered by two dihedrals; one with a horizontal edge defined by two planes passing through the upper and lower horizontal edges of the space for mounting the plate, the angles of which in relation to the horizontal are shown in Figure 1; the other with a perceptibly vertical edge defines by two planes passing through each side of the plate, the angles of which in relation to the median longitudinal plane of the vehicle are shown in figure 2.

I short, you fall foul in arguably three areas, perhaps two. Your license plate (section 2) does not protrude the distance of the length of the bike, obstructed by the exhaust cans. The licence plate (sec 3) does not meet angle requirements (it is facing downwards of greater than 15° incline) and finally and most importantly (sec 6) it is not visible in an angle of 30° from both sides of the bike when standing behind it nor is it visible at 30° above or 5° below from behind.

You could argue that the plate does satisfy section 2 as it is visable on the longitudinal plane (from the rear) but it's not a strong defense.

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