Walford v Miles  1 All ER 453, per Lord Ackner at page 460:
The reason why an agreement to negotiate, like an agreement to agree, is unenforceable, is simply because it lacks the necessary certainty… How can a court be expected to decide whether, subjectively, a proper reason existed for the termination of negotiations? The answer suggested depends upon whether the negotiations have been determined "in good faith". However the concept of a duty to carry on negotiations in good faith is inherently repugnant to the adversarial position of the parties when involved in negotiations. Each party to the negotiations is entitled to pursue his (or her) own interest, so long as he avoids making misrepresentations. To advance that interest he must be entitled, if he thinks it appropriate, to threaten to withdraw from further negotiations or to withdraw in fact, in the hope that the opposite party may seek to reopen the negotiations by offering him improved terms…A duty to negotiate in good faith is as unworkable in practice as it is inherently inconsistent with the position of a negotiating party. It is here that the uncertainty lies. In my judgment, while negotiations are in existence either party is entitled to withdraw from those negotiations, at any time and for any reason. There can be thus no obligation to continue to negotiate until there is a 'proper reason' to withdraw. Accordingly a bare agreement to negotiate has no legal content.