You did not read my bigger post showing that we pay the same price in usd with the americans with the fluctuation of the pound between $1.27 to $1.32 making the net value of the £600 card $633 - $658, and in USA the cheapest GTX 1080 goes for $649.99 while most of them $700.
(Net value = gross - vat)
Simple maths
I read all of your post. I commented on the part where you threw in 15% as "a healthy margin". Again, do you work in importing computer components, because that is way off from what I think. You are fond of saying "simple maths". But x + y can be simple and still wrong.
If you're going to make an argument then you can't have key parts of it just be hand-waved figures you feel are right based on no experience. Especially when they contradict what people who DO work in the industry are saying. Furthermore, your choices of pound values in your example: 1.27 and 1.32 are very poorly chosen and your argument ignores lag in prices as well. You say "simple", I say "simplistic".