The rise of China has made the first tier out of reach to countries with only 60-80 million people (such as the UK, France, and Germany). If the European powers combined - somehow, who knows how - then they would have the weight to be global powers and have more influence to move global affairs in directions the Europeans desire. With only 60 million people, the UK is destined to be a bit player.
One wonders why a neighboring second tier Eurasian power would want to disrupt European integration and the resulting arrival of a superpower on their doorstep. Stand together, or hang separately.