Have always worked in the rain since I started window cleaning and although I can't give exact figures, all I will say, is I have been cleaning windows for 15 years and If it didn't work I would not have continued cleaning in the rain for this long.
I would guess at most I have lost probably 10 customers over this time when they have really had a problem. I always say this. Yes I did clean them when it rained, however, what would you want me to do if I cleaned them at 9am and at 9.15am it poured down, am I expected to give you a refund? When you use a car wash do you take it back if it rains an hour later?
Generally, most customers accept this and understand you are very busy and don't have time to call back just because it rained, besides, I always end up joking with them. I explain they can watch the rain now through nice clean windows.
Be confident in your approach, this is your business and you can't be held back or accountable for the British Weather, unless it's torrential rain, keep earning!