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On average there is a 2" inch difference between US & UK...but remember the sizes here vary a lot.
http://www.fancydressnation.co.uk/acatalog/Si ze_Guide.html
You need to scroll to the bottom of this link for teen.
This should help
You can pick UK or US, and it works on your measurements rather than size. I am sure some uk shops just make the sizes up!
Let me know what you think
Now all these links are helpfull and I do not wish to confuse you even more, but in my closet I have jeans of all sizes, from 0 to 6. And they all fit just fine.
What I find helpfull is if brand you want to order actually has measurements in inches next to a size - sort of like the link above that swiral provided. The only thing is, sometimes I am a different size even by the same manufacturer. 
The trend for some time now is to lower the size number while the measurements stay the same. So what used to be, for example, 8 is now 6 and so on. I remember when I was a kid, I used to wear European number 38. Now, 15 years later, I am bigger and wear 34 or 36. Just crazy! :D
Anyway, best of luck.
