So when's the US going to merge with the rest of the countries in the continents of N/S America, so that it can catch up with China?
I don't know, but in the long term, US will have to join Europe and South America.
Right now, its economic, political and military power is unsurpassed, so it doesn't have to join anybody.
Europe, on the other hand, has to integrate. EU when taken as one entity has economy bigger than United States. When acting together, it matters in the world. In WTO, EU is represented by one representative, acting on behalf of the whole Union. When the US wants to negotiate about trade issues, they call Brussels, not national capitals.
It follows that if Europe wants to matter also in the field of hard power, it has to integrate more - and the nation states have to give up their veto in security and defense matters.
We can continue living in delusion of national sovereignty, but we will be marginalized. It's easy choice, actually.
Your numbers are way off - the UK is 60 million compared to the US' 300 million, so that's a factor of 5. And it's not clear that population, rather than military might or economic power, is the main criterion.
Ultimately, everything is derived from population. When China reaches the same level of development as the US, its economy will be 4 times as big, which will mean 4 times as much money for its armed forces, 4 times as much money for reaserch etc.
Smaller nation mattered in the past only because they were more advanced. Britain, relatively small nation, ruled over 1/3 of the planet.
This is not possible anymore. People have to understand that the world has changed. As of now, UK only matters as the closest US ally.
Any EU member state is too small to matter in the globalized world. If we don't integrate, we will be simply overlooked. Is that the future we want?