So basically what you guys are saying is... The yank engineers are wrong, solar panels DO need cleaning..... but no one has any data that proves why or how much improvement it makes.
Apart from....... they look nicer 
I have seen some measurable data from my last solar panel clean. I cleaned a third of an array which was split into three sections with the outputs recorded, on-line, in real time. We compared the before and after output figures and the third that I cleaned showed no measurable improvement over the two thirds I didn't clean. An adjoining property had a smaller array fitted at the same time and also not cleaned since construction. Their clean resulted in a measurable improvement of 30% !
The difference was that my section was in a marshy field, away from the road and trees and the other section was in a farmyard, on a barn, with tractors turning underneath, lots of dust and the presence of molasses that left black, sticky streaks all over. The summary is that it seems to be the TYPE of contamination not the amount - Dust on my panels was negligible but the more agricultural waste on the others was not. The average rain frequency for the UK is every other day - California, rather less - So dust in the UK isn't going to be anything like that of CA and by comparison only, the UK could be considered self cleaning under certain conditions.