your forgetting sales tax and VAT
every time a US launch price is mentioned it is always pre-sales tax (before tax just in case you don't get it)
therefore $399 pre-sales tax usually equates to £399 with VAT added.
the PS3 was £300 in japan and £425 in the uk is another example. the UK never gets it for the same price, due to the various different ways tax works in different countries as well as transportation costs, retailers mark up being different within different regions, etc.
therefore if it is $399 pre sales tax in the US, it is even more likely it will be £399 with VAT added in the UK, you just basically added more ammunition to the argument it will be £399.
e.g. a company selling it in the US for $399 pre sales tax, will most likely at the end of the day make just as much profit as a retailer in the UK selling it for £399 with VAT, this is just the way the system works. things are always cheaper in america because it's a completely different economic system.
Your also forgetting how much the exchange rates have changed since the last generation of consoles adding an extra £40+ on instantly at the same dollar/yen price.