Howabout A50 clearly states that the EU acts as a Union during negotiations, not as individual members:
http://www.lisbon-treaty.org/wcm/the-lisbon-treaty/treaty-on-European-union-and-comments/title-6-final-provisions/137-article-50.html
2. "A Member State which decides to withdraw shall notify the European Council of its intention.
^ In the light of the guidelines provided by the European Council, the Union shall negotiate and conclude an agreement with that State,^
setting out the arrangements for its withdrawal, taking account of the framework for its future relationship with the Union.
That agreement shall be negotiated in accordance with Article 218(3) of the Treaty on the Functioning of the European Union
. It shall be concluded on behalf of the Union by the Council, acting by a qualified majority, after obtaining the consent of the European Parliament."
Of course it is 27 vs 1, because only 1 country is leaving.
Naturally this puts the leaving country at a disadvantage of size,
plus the fact that by default it is losing its trading relationship with the EU, its 40 FTAs with 60 other countries and 750 other trade agreements,
its membership of agencies for essential services like flights ...
However, a cacophony of 27 negotiating teams, each demanding their pound of flesh, would likely have left the Uk without e.g. Gibraltar, fishing rights, any other goodies the individual members want...
and probably 10 years further down the line.