The Road Vehicles (Construction and Use) Regulations 1986
No. 1078 PART IIE View to the front Regulation 30
windscreens and windows MUST be kept clean and free from obstructions to vision.
The Construction and Use is very clear on this.
Now you can argue the toss about if your placement obstructs your view all you like but if in the opinion of a trained police officer it does you get a ticket.
From the current MOT inspection manual. Please note that the use of the words "Seriously Restrict" only applies to official stickers.
Windscreen Inspection Zone
Zone 'A is:
. in the swept area of the Windscreen
. 290mm wide
. centred on the centre of the steering
wheel.
'Official' stickers
'Official' stickers that are not readily removable - such as vehicle
licences, parking and access permits - are only a reason for rejection
if they seriously restrict the drivers view. 'Official' stickers
should be taken to be any sticker that is used in connection with
'road enforcement', 'security' or 'crime prevention' matters.
These include: Disabled Persons Badges/permits, Parking Permits.
Licences and Vehicle Anti-Theft Scheme stickers issued by a Policy
Authority.
Sun visors
Sunvisors fitted to the driver's side, which cannot be stowed
in the 'offscreen' position and which drop down obstructing the
view through the swept area of the windscreen are a reason for rejection.
A similar defect on the passenger side sunvisor is not a reason
for rejection.