You misunderstand my point. Even if, on the surface, they continue to ask what the "EU" thinks, they'll actually be asking (as they always have) what Merkel, Hollande, and (now) Theresa May think. Greece, Italy, Portugal, etc. don't have a say because they need to keep their heads down and continue to service their debt to the big 3 EU leaders, who clearly don't agree on much, even at the global level (war, trade agreements, etc).
Have you been to Africa? Have you seen the level of infrastructure (education, access to electricity and clean water) relative to violence (civil wars), corruption, and pillaging of resources (western countries, China) that continues to take place across even the most developed African nations?
The UK may not even exit the EU at the end of the day [1]. It's much too early to make such dire predictions for the future of UK growth. I prefer the simple or "naive" view that things will not change much.
[1] A referendum in the UK is not binding on the government of the day; it is advisory. The UK has a government with a slender majority (implying it will be tricky to ram things through parliament), it is looking for a new prime minister (and, possibly, a new leader of the opposition), the referendum result was close (the country is split) and no one seems certain who has the power to get the ball rolling by triggering the “infamous” Article 50.
EU will die its slow death, Brexit is not the cause, its the catalyst.
The EU sceptics may be smug now but the EU is not some selfserving idea. It's now more relevant than ever.
> Also, the UK is still the 5th largest economy in the world by GDP ($2.8T).
But relatively speaking, it's position is weakening year by year, as it's seeing anaemic growth compared to a large number of other economies.
More importantly, we're seeing the rise of more and more trading blocks, at a time when the UK has decided to stand outside. E.g. Africa is going to overtake the UK as a whole shortly. Hardly all that strange given we're talking about more countries than the EU. But ECOWAS - one of the regional pillars of the African Union - is on a trajectory to overtake the UK all on its own sometime in the next couple of decades, and with current growth it is likely more of them will.
It is pretty much inconceivable that the UK will remain 5th over the next decade, and it's looking likely that the UK won't even remain in the top ten over the next two decades, unless something happens to massively increase UK growth.