British and US pre-U systems are different, but the gap is smaller than the gap between other pairs of countries , partly because these two student populations often switch to the other system. In the excerpt below, all highlighting is mine.

Although there are no official equivalencies between the US and UK education systems, most UK and US educators agree that a US high school diploma (without AP courses) can be roughly compared to five GCSE (O-levels) passes at grade C and above. A US high school diploma with AP courses can be roughly compared to GCSEs plus A-levels.

So, for example, a British student holding GCSEs when he or she enters the US may be academically more advanced than US students of the same age. US students experiencing the British system for the first time at 15-18 years of age may find it more demanding than they had expected. Pupils who have been studying at top level private schools in the US may be at a much higher level than ones from the public school system and may find our level about the same as what they know back home. Some of our American students have also noted that the mathematics program demanded by the GCSE qualification is very different what they experienced at home:  By taking a more integrated approach (as opposed to the more rigid separation of different topics in math, such as separate courses in algebra and geometry), students were able to both review material they had already covered and explore new ideas without sacrificing extra time to multiple math courses.

Because the academic level of a US High School Diploma achieved without AP coursework can be as much as 2 or 3 years behind the British A-level standard, most UK universities will not accept American-educated students holding a high school diploma without any additional honors, such as AP test results or SAT subject tests. UK universities and the Universities and Colleges Admissions Services (UCAS) recognize AP exams as equivalent to A-levels. If a US student has taken A-level exams and obtained reasonable grades, then university entrance in the UK is not a problem.

If you return to the States with 2-4 A-levels with reasonable grades, you may find that you are issued credit and dispensed from some or even the majority of courses for the first year of university. This depends, of course, on your choice of university and your choice of subjects.