The UK system is reputed to be far less abusive than the US system. The US system put me through the frigging wringer.
(1) UK undergrad programs are a lot more focused, so students tend to be a lot further along before they start a PhD program.
(2) UK PhD students are funded for a fixed time 3(?) years, and then you're supposed to be done. That doesn't always happen, of course—I knew students who went on the dole to continue once their funding ended—but the system is focused on getting them out in that period. In the U.S., on the other hand, there seems to be a lot more variability; we've all heard the stories about CS grad students that take 10 years to get their degree...
It depends. Definitely doing a theory-based PhD can benefit from a European (not just UK) undergrad education, but in systems or implementation, American undergrads actually have an edge (everything else being equal, which is never true).
> PhD students are funded for a fixed time 3(?) years, and then you're supposed to be done.
Truth. I didn't know anyone to get out in 5 or 6 years, let alone three (in the American system).
I actually found in-major comp sci courses to be EASIER than the pelim courses I took before I got into the department.
To be honest, I've heard that UK universities were easy, even Cambridge/Oxford, compared to some of the tougher US universities. But I really have no clue!
In the sciences, people either do 4 year undergrad degrees followed by 4 year (usually funded) PhDs, or they do 3/2/3 years (undergrad/Masters/PhD). In the social sciences and humanities, 3/2/3 or 3/1/3 or 4/1/3 are normal, going from undergrad directly into PhD is higly unusual in these.
The US 5/6 years usually include a Masters degree, as far as I know, which is usually not the case in the UK.
We (US vs. UK PhDs) aren't that different, we compete at about the same level on completion (with typical personal variations, of course). Even the length of the PhD program doesn't seem to be as important as total length in field.
Oh, I by no means meant to disagree with this!
My point was that the programs themselves are differently structured, so comparing the actual time a PhD takes does not really work, since there are a number of possible combinations of undergrad/grad(/grad)-degrees that make up the length of an individual project. In the end, people will probably be at about the same stage, but that does not mean that going from undergrad straight into a PhD in the US and the UK work he same way. As a further example, in Germany, you were able to do just a PhD, as your only degree ever, until relatively recently. In the end, it probably wouldn't matter, since PhD is PhD (in a way), but your way there is hardly comparable!