Here's a scenario: Boris gets PM. European right uses vote as an excuse to make the changes to immigration and labour rights they'd love to to "neutralise"* their own far right - i.e. "reform" the EU some more.

Boris parachutes into Westminster, Union Jack streaming above him, victory in hand and either calls a new referendum or declares the old one null and void because circumstances have changed.

* i.e. pander shamelessly.