There's mutual benefit in the negotiations. It's not two people holding guns to each others heads, it's horse trading of what the other wants. Of course there will be a deal and it's not because we're "willing to walk away", it's because Britain has a lot to offer Europe just as Europe has a lot to offer us and a deal is a gain for both parties. Both the Conservatives and Labour know that. The biggest difference I think being that Corbyn is acknowledging it whilst May is trying to act like its a game of chicken for election purposes.
Of course there is mutual benefit, both parties have been clear on that. You're missing the point re: walking away - it has little to do with your analogy of holding a gun to each other.