If you are not taking A-levels, it will be really difficult to find an American uni that accepts other British qualifications. Also, the ones that routinely offer financial aid to international students are all highly academic, and 4-5 grades in GCSEs will not be strong enough.
If you plan to pay for the studies yourself, you may want to look at community colleges - they are 2-year colleges that have far more relaxed entry requirements and are much cheaper than unis. Also, most of them have agreements with local unis, and credits for those 2 years can be transferred to the universities, meaning you end up with a university degree after 2 years at community college + 2 years at university. At USA college day that is held each September in London they usually have at least one community college present, and you can go and chat to them.