To negotiate, there has to be two things. 1) something you want, but you're willing at least partially to give up and 2) a willingness to walk away from the deal.
If you're not willing to walk away from the deal, if you absolutely have to have the deal then you can't really negotiate, because your oppo knows that you have to give in.
We've put ourselves in that position. We can't threaten to leave, and we can't threaten to not leave. We're committed to leave, even before we pull article 50. And after we pull article 50, we're doubly committed to leave.
The negotiation with the EU will be as big a sham as Cameron's pre-referendum negotiation.
Of course, it will be presented as a success. "I hold in my hand a piece of paper" is a fine precedent.