No Cold Calling signs do NOT apply to window cleaners...it's just of those ancient laws that has never been annulled.
The law in question (as discussed in Parliament during the reign of George III and later enshrined into the law of the land) reads:
"Whereas vagrants, tinkers, peddlars and footpads may not be wearisome to the ears of the God fearing populace of this our noble country of England by the act of knocking on doors and vending either goods or services to said persons our most gracious and absolute King George III has given special dispensation to purveyors who are initiated in the ancient art of wyndow cleaning, be it by a bucket full of foul smelling urine the stench of which comes direct from Satans breath or the mystical wand known as the double-you eff pee method."
That is brilliant - it's almost worth printing out and handing to stroppy householders.
Imagine the scene:-
You: Wanna window cleaner, mate?
HH: Can't you read my good man? The sign on my door says "No door-to-door salesmen!"
You: It doesn't apply to window cleaners!
HH: It most certainly does! How dare you etc etc ...
You: Look; here is a copy of the ruling that is on the statute books (hands copy of Simon Knight's post above)
HH: (Starts to read) Umm! Errr ....
You: (Pegging it off down his path doublequick) I'll leave that with you ...
I bet most of them would laugh.