It's really barely comparable in execution, but the UK consistently ranks a fair bit ahead of the US in educational rankings. In US compulsory education, the curriculum is different from state to state, and often municipality to municipality. Study throughout compulsory education is more teacher-led, and independent study habits aren't focused on. As a result there's more of a focus on learning the information and nearly none on how to learn.
Typically it's a much broader curriculum, but nowhere near as deep. The end result is an education which is more or less similar in quality, though a bit ahead here in core and useful subjects.Higher education is different because of the differences in compulsory education. Bachelor degrees are four years, both because they need more time to get students learning independently, and because the breadth of learning is still fostered. There's a push for graduate to be well rounded in their education, so lots of classes are required which aren't related to the degree.
The idea of mapping GCSEs and A-Levels to the US model is a silly one. In the US, you really can't even map a high school diploma from one county to another in the same state. Of course universities in the UK won't accept a high school diploma out of hand for an entrance qualification: There's such a drastic variance in quality that it's really not even used in the US. The uneven quality of compulsory education means that entrance requirements hinge on nationally accredited entrance exams.
The scores in these exams will really be the biggest determining factor in getting the university one wants, except for competitive programmes. All universities will look over high school transcripts, but all it really tells you is how a student succeeded relative to his or her peers. In other words, it will identify a slacker, but not necessarily a highly educated applicant.
When you can finally start to match up US and UK educations is partway into higher education. A US Associate's degree (or 2-year degree) is quite comparable to a Cert-HE. Degrees above this are more or less globally viewed the same.