There is now a Hollaback UK site. You can find it here. In addition to that particular resource I urge you to go take a look at this thread located at The F Word, which invites women to post their experiences on street harassment, and this post from the Black Cat. Street harassment is not to be tolerated. Visit the links, read, learn, quit harassing women. What you may think is a compliment is often threatening or just downright rude. And don't deliberately threaten women.
While we're on the subject, here's an article explaining how to respond to those viral 'rape prevention' emails. My personal preferential response would be to educate people not to rape other people. But when I mention that I tend to get withering looks.
Here are sexual assualt prevention tips guruanteed to work.
Here is a guy's guide to approaching strange women without being maced.
Lastly, here is a link to a survey by the Campaign to End Rape which is asking for women's responses to rape (whether you have been attacked or not).
Now, go read.