Thanks for posting IANP. I agree with you, I think Cheesebiscuit is wayyyyyy off base here. And he even posts the message as if he encounters frying machines all the time. Then he even follows up with another post to try to dismiss this. Amazing...especially seeing that he IS COMPLETELY WRONG.
What a shame, I'm glad the original poster only lost 20 bucks. Always research the web for other answers and use a bit of common sense. I find most people lack it.
Anyways, IANP is absolutely correct, all you need to do is purchase the UK power supply. No need to waste money on a step down transformer. Especially given the watts this sucker will pull, that would be one expensive (and stupid) purchase.
Basically you just need to make sure of a few things. Check the back of your xbox and current power supply (that big ugly gray thing). Xbox has two different models, the slim and fat (the original fat version). They have different power supplies. Basically the ebay and amazon ads will spell this out for you. But I will provide some helpful TRUTHFUL information here.
On my xbox on the back side it states 12V==12,1A and 5V--1A. And on the power supply itself it states Input 100-127V-4.7A with output of 175W, 12v-14,2A 5V--1A.
So basically you just need to purchase something like this http://www.ebay.co.uk/itm/Official-Microsoft-Xbox-360-Power-Supply-Pack-175W-Brick-Adapter-UK-Power-Cord-/251101465715?pt=UK_Video_Games_Cables_and_Adaptors&hash=item3a76d04c73
And the ad clearly states the input is Input AC: 200V-240V ~ 2.5A, 47-63Hz, which is your UK power from the wall. And the output is your 175W which is what your Xbox needs. The only other thing to check is the design of the connector. In this add you will see the photo of the connectors, make sure yours is the same. If it's the 175w power supply then it should be.
That's it! For 15 pounds you have yourself a Xbox that can plug into the wall and play. It's kind of silly MSFT doesn't sell these power supplies that handle 100-220volt like pretty much every other electronic device I have. I guess they just assume you wont be traveling on holiday with your XBOX so they will save the cost to manufacturer a power supply that will run on both 100 and 220.
I hope this will help. I just wanted to further explain the logic so even a monkey would agree with what IanP and I are stating. I couldn't just let a thread sit open with complete bullshit. I have lived on 3 different continents for the past 8 years, I think that should qualify me for providing an answer you can rely on.