I believe that the screen washer jets on the Q5?s are actually heated. If you notice the ?tubes? going to the jets are fabric-wrapped. I think this wrapping covers both the inner plastic tube carrying the washer fluid and also a wire which carries the power to the heating element.
As far as I am aware there is no manual control (switch) for the heating of the jets. I assume there is some form of automatic thermostat which turns the heating on when the ambient temperature drops below a certain threshold.
Unfortunately the current harsh winter conditions in the UK coupled with wind-chill cooling could result in the fluid in the tubes leading up to the jets freezing unless quite a high concentration of washer fluid additive is used. I agree that the front headlight washers are probably partially insulated behind the plastic covers.
I recall that Audi also recommend the use of a screen-washer / de-ionised water mixture. This is to avoid calcination within the heated jets (aka furring up within a kettle) if hard tap water is used.