The thing with the UK system is we specialize a lot earlier. A 16 when you choose your A-levels you are already making choices, then you go to university to focus on one subject, there's no "choosing your major" after a year or two. And it's not even the case that the US system produces noticeably better generalists; UK people just pick that up as they go.
That's true, but as you say UK students aren't necessarily that tied down. I moved (in the UK) from a physics undergrad to research in machine learning and statistics. This sort of move is common.
True in my limited experience (EE not CS). The UK PhDs get a pretty well defined thesis topic and graduate on schedule.
The US PhDs don't have a clear finish line until less than a year from finishing.
The "joke" in the US is that if someone says they are six months from finishing it is really a year, if they say a year from finishing it is 2+ years, if they say more than a year they have no idea how much longer.