a couple of things propole are these the box gutters that i've highlighted?
If so would a deep swan neck not reach over them at an angle giving you the height to get over them from the ground? they don't look that deep??
if not i would use a scaff tower or picker, a scaff tower if working at a consistent height as its a lot cheaper, and it wouldn't take 2 guys no longer than 8 hours work.
we done one similar but ours was more green than that, at least you can see a bit of daylight through them windows our ones you couldn't.
it would be a lot easier to get above them to clean, you can see what your doing properly and that you can make sure all the algae would be flushed away properly. the undersides will probably be no where near as bad. start at 5 in the morning on a sunday have that cracked out by 1, if to many shoppers about, 2 sundays 5am till 9am.
£600- £700 if one day, if two days incur the cost of the plant hire
thats how i would clean this