Not too sure how this is by the letter of the law but I think I will see if anyone stops me and use Rain X, as Dan Lockwood suggested.
For the aero screen to be an aeroscreen and not need washers and wipers, the driver must be able to get satisfactory visibility without looking through it. I don't think the fact you normally look through it matters as long as you can look over it.
Not the sort of thing the average policeman will go anywhere near as construction and use issues are mindbogglingly complicated. Of course, you could get stopped by VOSA, but they generally are looking for overloaded lorries and tachograph offences.
