So I'll try to clarify. The power brick that comes with the computer itself is the same in the US and the UK. It is designed with a removable plug that plugs directly in to the wall, so you detach the US plug from the brick and attach the UK plug. It's a pretty cool, efficient design. The power brick itself works anywhere in the world- you just have to swap out the plugs.
What this means is that if you buy the Mac in the US, you get the US plugs. In the UK, you get UK plugs. In India, you get India plugs. etc.
So, since you'll be buying in the US, you'll get the US plug, and have to buy the UK plug. You can get the Apple World Traveler kit, which includes the entire set of plugs for the power brick, including the UK, for $40:
http://store.apple.com/1-800-MY-APPLE/WebObjects/AppleStore.woa/wa/RSLID?mco=47FFCC51&nplm=M8794G%2FB
One common complaint with this is that you can not buy any one of these individually from apple. You can't buy just the UK plug. You get em all. There is no individual packing.
[These are usually in stock in the stores, so if you're going to one of the stores in NYC you can just grab one when you purchase the computer.]
You can try to buy just the UK plug on ebay or in the UK itself, though I have absolutely no experience with that.
Good luck!