British Telecom have a Privacy Policy. I reached this page from the BT Webwise page, so presumably it applies to their proposed arrangement with Phorm.
We may share your personal information with other companies so that they can contact you with details of other products or services you may be interested in. We will only do this if you have agreed to this and where the companies agree to use your personal information for that purpose only. If you have agreed to receive information about products and services from another company and later decide not to you will need to contact that company yourself to let them know. You can of course ask us not to continue to provide your personal information to any more companies in future.
From this, I would assume that writing to BT and requesting they do not harvest my web browsing habits (opting out from webwise does not appear to stop this happening) would really opt me out from their data pimping.