I think you're dealing with a combination of two things: One - yes, unfortunately there are quite a number of Americans who are not very familiar with World history or fact...so quite a few have never really understood any distinction between England, UK, or GB. They all essentially mean the same thing to quite a few Americans. Scotland for a few might actually be identifiable entirely on its own, with some not even affiliating it with any of the three names, mostly because Scotland has its own special appeal to many Americans. Ireland too is oft thought of as a single location, with likely more than a few Americans not knowing there is a Northern Ireland, and that it is any different from the Republic of Ireland.
The other factor you are likely dealing with is a lazy language usage, and use of the most familiar name as a catch-all for anything similar. You will often hear people use the word 'Coke' to refer to a soda/pop, or 'Kleenex' to refer to any tissue paper. Similarly, quite a few people lazily use 'England' as a general familiar term for anything landing in the general vicinity of the United Kingdom...sadly even sometimes including the Republic of Ireland under the term. Some of these folks might actually know the difference between England and the United Kingdom, just as they know the difference between Coke and Pepsi, and if challenged on the subject, could properly elucidate. However, once relaxed and not concentrating, they would likely lapse back into their lazy usage.
Great Britain is an entirely different issue - that one would challenge many Americans, who never studied British history, and would likely not understand the differences between the initial union of the Kingdom of England with the Kingdom of Scotland to form Great Britain, versus the addition of the English-controlled Kingdom of Ireland some century later to form the United Kingdom. Nor would they likely be familiar with the geography of the region, therefore unfamiliar with the fact that the island itself, upon which sits most of England, Wales, and Scotland, is called Great Britain.
As for Epcot's pavilion...They seem to have primarily concentrated on English styles - with the main street being occupied by buildings meant to represent different centuries of architectural style in each building. A hint of countryside cottage design, and a hint of garden thrown in, the rest of the United Kingdom is mostly represented by the stores, with various trinkets, decor, and goods from Wales, Scotland, England, and Northern Ireland. Pretty hard to fit all of UK's history into one small pavilion...so they've done admirably well given that. I'm sure the Chinese find that pavilion rather lacking in representation of the 33 diverse regions and 56+ ethnic groups spanning an architectural style and history of over 4,000 years...but there's only so much room in the pavilion space!
