It's right in the contract that member states have the freedom to leave.
So tl;dr; Britain should stay in the EU because #remain folks want to start a trade war with them to punish them for leaving? And that maybe this will result in an actual hot war? You seem to be doubling down on my abusive spouse metaphor.
I'm a huge free trader and open borders type. (And also a yank in India, so no skin in the game.) But listening to the arguments of #remain folks makes me think the UK is better off on their own. With friends like these, who needs enemies?