shouldn't assume they aren't paying tax just because they only charge £3.00, I know lots of legitimate window cleaners that charge that and pay their tax and some of them are pretty good shiners as well.
Don't get me wrong I wish they would put their prices up but it is up to them at the end of the day.
and I Know the lad that cleans my street, he pays tax, has insurance and a license and still only charges £3.50 a house so don't tell me it can't be done, he has a newish 4x4 bought and paid for, lives in a nice house in a nice area, oh and he does it all off a ladder