Make that a
chain=dstnat src-address=80.x.x.x protocol=tcp dst-port=80 action=dst-nat to-addresses=80.x.x.y to-ports=8001
(include the dst-port paramter) or your webserver will see all kinds of TCP traffic redirected to it which it cannot really handle (SMTP, FTP, ...).
If you made sure you have a webserver running on port 8001 at the address you redirect to, also take care to configure it so every errorpage (especially the 404 one) will show your "we're missing money from you"-page. (As your client will perhaps request http://www.someaddress.com/this/other/page.html
- and your webserver won't have that special file...)
If that doesn't work, try to explain better what acutally happens ("it won't redirect, but they do not have internet"?).