Actually UK is not a good sample for online games.
For example League of Legends, the biggest online game of the last 4 or so years, has VERY few UK players (% of population) compared to almost any other EU country.
Ohhh a numbers game this should be interesting...
Unfortunately there are other variables that make the UK a terrible sample, though. As others have stated, your original post only shows retail sales of the game. Most people bought digitally unless they wanted the physical CE. This further poses a problem because if I remember right, the CE was grossly over-priced in the UK (you can correct me if I'm wrong and they fixed it) as SE never actually performed the conversion of dollars to pounds and just deemed that $130 dollars and 130 pounds were the same thing. As such, the physical CE (again the only reason most people bought retail) is likely to see even lower sales in the UK than in the US or Japan.
Clearly you weren't on Aether data center.
On that note, Heavensward PS4 and PC physical copies were completely sold out of the local Gamestop (Georgia, in the US) as of last week. (A friend was there for something else and inquired.) The only thing left is PS3 physical copies, because nobody loves the PS3 any more.
The game had restricted physical sales also. At one stage we didn't even know if a physical CE was going to be offered on sale in Australia and when it finally was it was $300.
So retail statistics probably aren't the best to use since it wasn't made as available at retail as it was digitally.
Yup, I think the physical copy is treated like the way PC games are. (since you see no one selling used PC games...which means they don't take em)
Yes true, the only copy of ARR I see in one location of GameStop is for PS3...PC and PS4 are sold out...I guess if I ever do upgrade to a PS4 I'm gonna have to go digital for that...and probably the expansion. (lol I think from that particular store I got the very last PC copy of ARR - I have the uncanny ability to get last copies of things)
I'm not sure about the retail vs digital sales, but playing ESO I know that they've only got two servers for the PS4 version one NA and one EU, that leads me to believe that it's probably not quite as big as ffxiv, though I could be mistaken.