I guess you're right. It's still not £100, though.
The US is unsurprisingly getting a much better deal than us. The difference between the two is only £20-£30.
But yes Nexus, you really should have waited. If you haven't opened it yet I highly recommend you return it, as if the law in your country is anything like ours you're perfectly entitled to do that.
I'm guessing the GTS cards will just be phased out as of the GT's release, so the retailer was probably trying to offload one onto you before knowledge of their obsolescence becomes widespread.
There is pretty much no incentive for anyone to buy one of the GTS cards. They're more expensive, they have inferior performance, and they run hotter. You were warned, Nexus...