It took ages to find the support page, so I'm keeping a note here:

(This article only applies to you if you have an old BT Openworld website.)

To log in to your old BT Openworld web space, please:

1. Type the following into the address bar in Internet Explorer: ftp://ftp.btinternet.com
2. Click on file menu at the top of the Internet Explorer window
3. Select Login as
4. Enter your username and password in the pop-up box that appears
5. Click OK and you are now logged into your web space.



Using Firefox

FTP add-on for Firefox 2 installed.