Here's a link to it: https://www.ridersite.com/viewtopic.php?t=4409
The relevant rules on visibility seem to be:
the character width for our bikes is 44mm.(2) A registration plate must be fixed on the rear of—
(a) the vehicle, or
(b) where the vehicle is towing a trailer, the trailer, or
(c) where the vehicle is towing more than one trailer, the rearmost trailer.
........
(4) Except as provided in paragraph (3) the plate must be fixed in the manner required by
paragraph (5).
(5) This paragraph requires the plate to be fixed—
(a) vertically or, where that is not reasonably practicable, in a position as close to the vertical as is reasonably practicable, and
(b) in such a position that in normal daylight the characters of the registration mark are easily distinguishable from every part of a relevant area having the diagonal length specified in paragraph (6).
(6) The diagonal length of the relevant area is—
(a) in the case of a mark having characters the width of which is at least 57 millimetres, 22 metres,
(b) in the case of a mark having characters the width of which is 50 millimetres, 21.5 metres,
(c) in the case of a mark having characters the width of which is 44 millimetres, 18 metres.
So ... It should be fixed on the back of the vehicle ..... should be vertically mounted ..... should be visible from every part of an area behind the bike that has a diagonal of 18 metres - so I guess the most favourable interpretation is of a square with 18m diagonals with one of the 4 corners placed on the back of the bike.
..... arguing for absolute compliance of nooj's plate may be a bit of a challenge ......