I'd really like to know where they came up with those numbers. For instance, the graph shows the USA near zero until a few centuries ago, but still non-zero, particularly around 1000CE where it goes up a little before going back down until it begins its steady climb just after 1700. There was no "USA" in 1000CE.

And seriously, how they they possible come close to quantifying relative GDPs in ancient times, or even as early as the 1700s or 1800s?

Realistically, the colonial era and Brexit were symptoms of two totally different issues. The greater British empire disintegrated in much the same way as those belonging to the Dutch, French, and Spanish. Brexit was the result of the EU being very dependent financially on England (and Germany etc) but it wasn't an issue of England disintegrating, it was the EU falling apart (and if you like a little conspiracy for breakfast, https://en.wikipedia.org/wiki/Foundations_of_Geopolitics).