I know it sounds cocky, but sometimes when cleaning in light rain, we get new custies coming over to ask us if they can be put on the round, and could we start today. Now and then, one will stick their head out of the door and say "You're not going to clean 'em in this are you?".
They aren't just dropped, they get replaced. In other words, as soon as you notice you have a custy like that, go get 2 others to replace them. Doesn't matter how long it takes to replace them, just make a point of doing it.
Personally, I hate working in medium to heavy rain, I will finish a job once I've started but otherwise I just have a day to do other work.
I have heard of Appointment only work from some guys in UK, but it's rare I think. It's normal here in Germany and in Holland where Pingu is though.
Another point is, if everyone thought it was silly to clean in the rain, all the Scotish guys would be out of business. That's not meant as a joke either.