Think about it like this, no one pays you when you're sick, on bank holidays or any other holiday for that matter. Look at Christmas, two weeks without income and then if the weather is bad you could loose more days still. Then you've got other things to pay for, some that you have'nt allowed for, an accident, a blown engine, kit to replace.
Every year I see a new lot of window cleaners come and go. Why? there's loads of work, its simply that they did not charge enough to sustain themselves.
Charge as much as you think the customer can stand, provide the highest level of service and reduce the time on each job by being efficient and well practiced at your skill. This way you will push your hourly rate up to £40 plus where it should be!
Tom