Print it, Sign it, Scan it. I scanned my to pdf files.
Then send as an attachment via email with your user name and store name and 'w8-ben' in the subject line and body of the email.
That should do it, it's not neccesary to tick Part 3.
When it's processed you will see 'We have received and processed your form. Thank you!' on your payment settings page.
Check this thread
http://forum.zazzle.com/intl/british_and_w8ben_form_help