Unfortunately, it doesn't seem to matter if you opt in or out.
The system only appears to check for the presence of the opt out cookie *after* your browser request has gone through the profiler. So all you actually opt out of are the targeted ads. Phorm still get the marketing data.
Was with BT, now have the MACs for my 2 accounts and actively looking for a principled ISP.
The closest real world analogy to what they are doing is having your postman open all your mail, read it through for keywords and seal it back up so he can better target you with more relevant junk mail. Obviously totally acceptable
Make no mistake, even if you have logged in to PPRuNe, any posts you make here are on an unencrypted (non-HTTPS) page and would be included in the profiler, as would any mail you checked via web-mail.
Edited to include last 2 paras