I'm sure someone more in the know will come along and correct me if I'm slightly off, but from what I know...
To get into US colleges, you need to complete a High School diploma equivalent and possibly advanced placement classes (APs). This roughly equates to GCSEs + A-Levels.
There's actually some debate stating British A-Levels are significantly harder to US APs e.g. since they include Multiple Choice Questions and the occasional short answer prose questions and essay questions.
I believe you need APs to get into the Ivy League Colleges, but can get into the less prestigious ones without. You need A-Levels minimum to get into ANY British Unis... That says something about the equivalency :P