Yes 'flecc', that too is my understanding of how the expression GB came to pass.The word Great was added to Britain to reflect the enlarged area of the country when Scotland joined into union with Britain (England and Wales), that's all. Therefore the word Great does not in any way reflect status, in the sense of standing in the world.
Deviating from that matter somewhat but still loosely connected to the matter of 'Brexit', I find it interesting to conjecture with the German situation in Europe, the EU to be more precise I suppose. WW2 may well have been Hitler's attempt to create the 'Third Reich' which everybody has heard of but few understand properly.
The dream of Hitler and millions of Nazi supporters to create that third great kingdom or empire failed to materialise due to the German failure to actually win the second world war. Amazingly, just 70 years or so after that defeat, Germany is now arguably the strongest and most important state in Europe so, given the stated aim of EU leaders to progress the EU into a United States of Europe, it could be described as the achievement finally of that 'Third Reich'.
Perhaps it is not exactly what the little Corporal had in mind but I'm sure most Germans will be happy to be where they are today without any further casualties or destruction of their fine cities.
I suppose I should mention that a little internet research reveals that the 'First Reich' to the Germans was the Holy Roman empire and the 'Second Reich' is generally described as the period when many German independent states were united under Kaiser Wilhelm 1 with his Chancellor, Bismarck in 1871 until the end of WW1 when German lost much of its territory.
Hitler's dream was of a 'Reich' that would last for a thousand years and maybe we have just seen the first few decades of that dream come to fruition. Those English men and women claiming to be of good Anglo-Saxon stock shouldn't be too disappointed about Germany being the strongest nation in the EU as those Angles and Saxons all started out in the area of Europe we know today as Denmark and Germany. It's all in the history books.
Tom