I came across this gem today. I had always thought that subsidiarity meant that if something could be done better at the national or regional level, rather than at the EU level, then it would be done at national or regional level. No. Apparently that was me being credulous!
Article 5 of the Treaty on European Union says this: “Under the principle of subsidiarity, in areas which do not fall within its exclusive competence, the Union shall act only if and in so far as the objectives of the proposed action cannot be sufficiently achieved by the Member States, either at central level or at regional and local level, but can rather, by reason of the scale or effects of the proposed action, be better achieved at Union level.”
So, if something falls within the competence of the EU, such as health and safety legislation, then any action must happen at the EU level. On the other hand, if something does not fall within the competence of the EU, such as defence, then the EU may act if it decides that the proposed action cannot be sufficiently achieved by member states.
The wording is carefully asymmetric, in favour of action at the EU level. The UK is so much better off out!