http://www.opsi.gov.uk/si/si2001/20010561.htm#6



2) Except as provided in paragraph (5), a registration plate must be fixed on the front of the vehicle in the manner required by paragraph (3).

(3) 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,

(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 (4).


if you want the law look here ...

http://www.opsi.gov.uk/