ps, is it because you spotted those mountless numberplate GTi fronts on ebay ?
heres the regs on placement (and i think these will be the ones to get you in trouble if the law see fit to go that far)
(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).