"Ooo, that's an interesting 'largely'. I don't remember that being in the campaigning."

I would have thought it obvious that no country can be truly independent unless it exists in total isolation. I am sure anyone could work that out. Perhaps you require a campaign to aid buttering your toast of a morning.
Over flights and air traffic management is one obvious area where agreement is required. This is not ceding Control, it is a case of reaching agreement.