PhD programs in the UK (and rest of Europe) take around 3 to 4 years to complete.
After a PhD in the UK, students generally go on to their postdoctoral research.
In the US, a PhD may take up to 5 or 6 years.
After a PhD in the US, students tend to go directly from graduation to academia or research jobs without a postdoc.
In many UK (and European) universities, there are firm guidelines on just how long a PhD takes and those are more important than individual decisions by a student’s advisers. In comparison, in the US, some students can fly through their PhD in 3 years with tremendous amounts of research, while others can take as long as 8 to 10 years to complete their PhD.
There are different systems within Europe.
In Sweden and other Scandinavian countries, a PhD takes 4 to 5 years and includes additional teaching duties. Students in these schools are considered as employees. They receive monthly salaries which are comparable to the salaries earned by graduate students working in various industries and are taxable as well. A PhD student is allowed to either present or attend at least one conference anywhere in the world, expenses for which are taken care of by the research group.
In Germany, a 4-year PhD is considered too long and funding might not be available after the first three years of the PhD program.