It was clear this was a one time event with no turning back.
Not so. Here are three articles from the Lisbon Treaty:
Article 505. If a State which has withdrawn from the Union asks to rejoin, its request shall be subject to the procedure referred to in Article 49.
Article 49Any European State which respects the values referred to in Article 2 and is committed to promoting them may apply to become a member of the Union.
Article 2The Union is founded on the values of respect for human dignity, freedom, democracy, equality, the rule of law and respect for human rights, including the rights of persons belonging to minorities. These values are common to the Member States in a society in which pluralism, non-discrimination, tolerance, justice, solidarity and equality between women and men prevail.
It may not be desirable, it may be cumbersome and the outcome tenuous - but the process for re-newed membership exists.