The rules state it should be on the Nearside (against the kerb)
Doesn’t state where on the nearside, but I’ve known pedantic policemen give FPN’s for it being at the top of the nearside.
Exhibition of vehicle and nil licences
6. – <snip>
(3) Each such licence shall be fixed to the vehicle in a holder sufficient to protect the licence from the weather to which it would otherwise be exposed.
(4) The licence shall be exhibited on the vehicle –
(a) in the case of an invalid vehicle, tricycle or bicycle, other than in a case specified in sub-paragraph (b) or © of this paragraph, on the near side of the vehicle;
(b) in the case of a bicycle drawing a side-car or to which a side-car is attached, on the near side of the handlebars of the bicycle or on the near side of the side-car;
© in the case of any vehicle fitted with a glass windscreen in front of the driver extending across the vehicle to its near side, on or adjacent to the near side of the windscreen;
(d) in the case of any other vehicle –
(i) if the vehicle is fitted with a driver’s cab containing a near side window, on that window; or
(ii) on the near side of the vehicle in front of the driver’s seat and not less than 760 mm and not more than 1.8 metres above the surface of the road.
(5) In each case referred to in paragraph (4), the licence shall be so exhibited that all the particulars on the licence are clearly visible in daylight from the near side of the road.
http://www.legislation.gov.uk/uksi/2002/2742/regulation/6/made