Only detail on gov website says:
Displaying your tax disc
Your vehicle tax disc must be displayed on the passenger side (kerb side) of the vehicle’s windscreen. If there’s no windscreen or you have a motorcycle or sidecar, you should display the tax disc on the kerb side of the vehicle.
Advice generally given is that it must be located somewhere which is outside the sweep area of the windscreen wipers. That way, you won't be creating a blindspot.
From the The Road Vehicles (Registration and Licensing) Regulations 2002
(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;
(c) 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.
HTH
BTW - my Saab has the tax disc on the RHS (don't ask) for the last year with no problems. Also, my wife's beetle had the tax disc attached to the dashboard, rather than the windscreen, for 3 years and was never picked up...