Asking?

You’re setting yourself up to lose work.

Show up, crack on, earn. If they turn you away you will have been turned away from one but cleaned 9.

If you keep power tools or other value items in van, people are more likely to steal those than the van.

I think all w/c,s should work in moderate rain to get rid of this nonsense 1960's  idea that rain dirty s windows.
Tell customers to google it ,if they don't believe  it
The ones that say give it a miss,tell them that means we would  be cleaning twice as much dirt for half as much money...and what if it's raining next month?
Boss them or loss them

Worst winter in my 21 yrs shining... It doesn't help that the forecasts are so unreliable, if you could trust that it was going to pee it down all day you could  mentally relax and make the most of the time off but instead your looking out of the window every 10 mins thinking "it's stopped, I ought to be out there". Cleaning in the rain round here is a surefire way to lose customers, I'm in the driest (usually) part of the counrty and  theres  little sympathy for turning up  when it's raining. I will ask sometimes if they mind in light rain before starting but  you can tell in their voice that even those that do  say yes aren't overjoyed. I totally get wc on the west side of the country having to be bit  more hard faced about it, you have to earn a  living, but round here there's usually plenty of dry days to catch up on. But not so far this year, wettest I've ever known it.

The issue isnt that no one will have them done in the rain its that youve had 3 or 4 people moan and you see that as no one wants them doing. Youve taken the opinion of a few customers and got it in your head that you will lose all your work.  So now you dont work in the rain and this makes it worse with customers thinking you wont be out if its raining. 

You asking for their permission compunds this even more, you arent sure so they arent sure. You dont ask customers if you can clean their windows you tell them you will be round tomorrow to clean them . If they question the weather you explain why it wont affect them and they can leave the round if they arent happy. Youve basically created your own problem.

If you only clean 90% of your round this month and the rest skipped or you dumped them isnt that better than cleaning only 50% of it because you were too scared to work in the rain, explain to people, offer a rain guarantee and ultimately start replacing the ones who moan.

You cant run a reliable, regular window cleaning round in the UK like that, it rains something like 160 days a year on average!

Worst winter in my 21 yrs shining... It doesn't help that the forecasts are so unreliable, if you could trust that it was going to pee it down all day you could  mentally relax and make the most of the time off but instead your looking out of the window every 10 mins thinking "it's stopped, I ought to be out there". Cleaning in the rain round here is a surefire way to lose customers, I'm in the driest (usually) part of the counrty and  theres  little sympathy for turning up  when it's raining. I will ask sometimes if they mind in light rain before starting but  you can tell in their voice that even those that do  say yes aren't overjoyed. I totally get wc on the west side of the country having to be bit  more hard faced about it, you have to earn a  living, but round here there's usually plenty of dry days to catch up on. But not so far this year, wettest I've ever known it.

This is not the way to run a window cleaning business IMO.

Just go out and get it done whatever the weather and only have days off in the very worst of weather.

You ve got to be firm but polite with your customers. The weather is changing all the time.

You shouldn't be asking them either just crack on.