I would disagree that A-levels are equivalent to AP exams from my research.
I think A-levels are more like actual college-level coursework here in the US, helping to explain why an undergraduate degree often takes 4 or 5 years here rather than just 3. I've heard of UK students being given college credit for their A-levels here. The structure of A-levels and sixth form in general reminds me a lot of a community college.
I mean, I notice that A-levels are split into two parts usually, and one part is covered per year. At a community college, I would take English 1301 and English 1302 in order to meet state requirements in language. This is covered in one year over two semesters, but I suspect that A-levels take longer because it's material presented to younger students. Several classes are split into two levels, and I notice that several AP courses only count for the first level. In other words, AP English might get you out of 1301, but not 1302. Which is a lot like saying that AP English is an AS level qualification.
The main difference is you're required to take everything. You have to take English, History, Speech, Government, Art Appreciation, etc. You have a lot of general education requirements that have to be met at both the high school and college level, no matter what you want to study. So students probably take so many AP classes in order to shorten the amount of time they spend on general education requirements, rather than to prepare for a particular field of study. But it only covers the first half, and you still spend your first year of university finishing up general education requirements rather than studying the subject you want to go into.
Now, having studied both systems, there does seem to be one way in which the British system is better. It's much more focused. You're encouraged to discover your talents/strengths and develop them, rather than meet a minimum standard in everything in order to be allowed to get anywhere. For instance, I wasn't allowed to take advanced classes until my senior year because I was behind in Maths, and they keep the gifted students separate from the normal students. They didn't keep me in remedial classes because I made progress quickly, but I could only take normal classes because I wasn't as gifted in Maths as I was in everything else. The PSAT really discouraged me, because while I made great scores in Reading and Writing, the Maths section brought my composite score down so low that I knew I wouldn't be going to a good university. The pattern here is that you're expected to be good at everything.
Now, it seems like by the time British people start on A-levels, they're allowed to choose what they want to study. Correct me if I'm wrong here. So, under your system... people are allowed to focus on subjects they find interesting and in which they have talent. So someone could take A-levels in humanities subjects like English and History, and not worry about universities looking at their Maths and Science skills, even if they're applying to a prestigious university like Cambridge. There's also no need to focus on composite SAT scores in Reading, Writing, or Maths... so if you're terrible at Maths, you just don't take it.
Both systems have their positives and negatives. American schools are easier in terms of difficulty, but harder because you're forced to be well-rounded and have some talent in everything before you're considered university material. British schools are harder in terms of difficulty, but easier because you're allowed to focus on subjects that interest you enough to handle that level of difficulty.
If an American changes their mind about what they want to major in, chances are they'll already have some foundation in any new subject and won't be starting over entirely. But if a British person does the same, they have to start all over again.
Another major downside to the US system is GPA. Every class you've ever taken, no matter what it's in, counts against this GPA equally. They look at your weighted average of all grades in all classes, and one or two subjects can bring it down even if you've done well in everything else. But it seems that with A-levels, if you make one bad grade in a subject, the other two grades are looked at independently, and that can potentially help you if the course you're interested in doesn't have much to do with the subject you did poorly in.
So, it really depends on what kind of person you are, as to whether the American or British system is better. The American system is better if you're a jack of all trades and make similar grades in all subjects, but punishes you if you're bad at anything in particular. The British system is better if you're very good at the subject you're pursuing, but punishes you if you try to do something you're not interested in and end up failing at it. There's not as much of a safety net of general education to help you with a Plan B if you end up changing your mind or doing really badly in the subjects you're interested in.
I will say that for me, the American system gave me an inferiority complex around Maths. I've spent years of my life feeling like I was a stupid person regardless of my skill in other subjects. I ended up forcing myself to learn Calculus later in life simply so that I wouldn't feel stupid, rather than because I wanted to study it. I feel like the emphasis has always been on what I can't do rather than what I can do.