From wiki.

"Article 42.2 provides for complete integration, which would require unanimity in the European Council of heads of state or government and has as such been blocked by the United Kingdom, which is the main opponent of EU defence integration, in particular. (The United Kingdom is however scheduled to withdraw from the union in 2019.)"

Yeah, yeah, blocked, vetoed, whatevvah. Obstinately the same result. Once we are out we can't block/veto it any more and it will happen. And the reason they don't invite us to meetings any more is that they don't want a country that's leaving to be involved in future decision-making. Duh. If we stayed in it would never happen. That's the constitution. Unanimity or not at all. Unintended consequences ahead, eh?