If the UK only exercises its right to withdraw from the EU under article 50(1) without negotiating any agreement, there is no need for any “agreement”.
Thus, as provided in Article 50(1), a Member State may withdraw without any other conditions “in accordance with its own constitutional arrangements”.
Since the United Kingdom is singularly fortunate in this situation that there is no written constitution, the UK is clearly free to act in any way it sees fit and proper.
It is important to stress that there is no need for any agreement at all with the EU. (The UK is also fortunate not to have the euro as a withdrawal for the eurozone states is much more complicated).