I'm pretty sure it's illegal to 'mailshot' anyone/businesses at random using e-mail. You only can do this if you have some kind of business connection to them in the first place.
I'm more than 'pretty sure' to be honest.
I doubt anyone would get annoyed enough to prosecute a window cleaner doing this, but I'm just saying it's illegal. I've done things that are illegal or unlawful (not sure which). I think I drove at 31 mph in a 30 mph zone today.
P.S. The difference between illegal and unlawful is that one is against the law, and the other is a sick bird.