JET = Joint European Torus

It was a coalition of all EU countries. Switzerland (non EU) joined later and UK quit the coalition with Brexit but continued to see this project through.

Their decision to break with the coalition is stupid, but I don't see any reason why they can't exchange knowledge in the future too.

The JET was basically a result of European scientists being invited to USSR to verify their results. The world of science is often a lot more open than the political that way.