I recommend the anonymous comment starting: It's unfair to characterize a UK PhD as "a good US Master's thesis"... http://matt-welsh.blogspot.com/2010/09/so-you-want-to-go-to-... — it's pretty fair and accurate.

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.

I'm pretty sure people in the UK used to say exactly the same thing about US PhDs!

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.