If BT have put a different web page at the same address as the old one, there's nothing you can do about it. The old page no longer exists.
I've never really understood why anyone should want any particular page as their home page (unless, for example, they regularly log into their e-mail via a web interface). My browser is set to open with a completely blank page, which saves time downloading a page I might not want to use.
As stated, you can't access a page which no longer exists. However, you can select any current web page as your home page. Simply go to the page you'd like to use, then click on Tools > Internet Options. You should find that the 'General' tab is already selected. (If not, click on it). In the top section of the panel, which determines your home page settings, click on 'Use Current', then on 'Apply' and 'OK'. (That assumes that you're using Internet Explorer, but the instructions for Firefox are basically the same).
Chris