The "European Union" as an entity came into existence at the time of the Maastricht Treaty (aka Treaty on European Union), and the UK was a signatory to that treaty. That was in 1992.
Could you explain why you think 2009 is the relevant date?
While we did have a referendum to join the EEC, I maintain that the EEC served a very different purpose (being a trade based agreement) to the EU.
The Treaty of Maastricht was passed by parliament, though was never put to the public vote.
The subsequent treaty of Lisbon and consolidation of the EU had no public vote either.
Brexit was a public vote.
It would be interesting to see what would have happened if the other two had been put to the vote