Matheous-2 - If a vehicle has a tinted strip along the top of the windscreen, then probbly displaying a disc in that area may render it 'not clearly visible' from the outside of the vehicle.
''Vehicle registration and licensing is governed by the Vehicle Excise and Registration Act 1994 (as amended) (VERA). Regulation 6 of the Road Vehicles (Registration and Licensing) Regulations 2002, which are made under the auspices of the 1994 Act, states that the licence shall be exhibited on the vehicle, 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. The Regulation further states that 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.''