Found this: for front plates
(3) This paragraph requires the plate to be fixed -
() vertically or, where that is not reasonably practicable, in a position as close to the vertical as is reasonably practicable,
() 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 (4).
(4) The diagonal length of the relevant area is -
() in the case of a mark having characters the width of which is at least 57 millimetres, 22 metres,
() in the case of a mark having characters the width of which is 50 millimetres, 21.5 metres,
() in the case of a mark having characters the width of which is 44 millimetres, 18 metres.