A further word of caution!!
Whatever you do, don't underprice!
Don't price cheap just to get the round up and running, if you do so, getting those prices up to a reasonable level once you start to get busy is a major headache!
If you think you would be happy with an income of say...£350 per week, then the hourly rate you will need to work off will need to be up around £16 per hour rather than £8.75 per hour.
For £500 a week you'll need to be up around £25 per hour.
I don't know how quick you are at the moment, but once you are experienced enough, you should be able to clean a standard 3 bed semi in under 20 minutes, to clean 3 an hour in a reasonable close knit bunch of accounts should be more than possible.
So that is roughly 12 windows, standard windows, UPVC.
In a 5 day week you will work nothing like 40 hours, you'll much closer to 30 or less.
Once you take into account all the non procuctive hours (Tea Breaks, chatting to customers, bad weather, travelling between accounts, late starts, early finishes, sickness and so on)
Well, you will be shocked at just how few hours you actually spend CLEANING windows.
And as the others have replied, all areas have window cleaners, usually several of them, but not ALL houses have them
Knock on doors, deliver leaflets, may the force be with you
Regards,
Ian