It's not about hostility. My remark about peace was meant to highlight an important advantage of EU membership: by participating in a permanent organizational framework, countries in the EU are able to settle differences and conflicts of interest diplomatically before they can lead to a crisis. Therefore, it's almost unthinkable that two EU member states could go to war with each other even under very difficult circumstances that might arise sometime in the future. It's all about building trust, furthering (and building) common interests.

However, EU member states are not allowed to participate in any free trade- or open border- agreements with countries that are not part of the EU, EFTA, CEFTA, oversea territories or candidate countries on their own, so that's what I meant by isolation (CEFTA will end when all other states in the region join the EU). In this age, it hurts small states economically not to be part of a trade bloc. That's why they're everywhere. World map of blocs

Considering the examples you cited, Hong Kong- UK was essentially a colonial relationship. As for South Korea and Cuba, I don't think their special relationships worked out well for these countries in the long term except for providing military security in the Cold War era. But it is over, isn't it?

"If you know your enemies and know yourself, you will not be imperiled in a hundred battles." Sun Tzu