Depends on the school. It's becoming increasingly prevalent in the UK for students to enter PhD programs with a master's degree, especially since many undergrad programs are now (in American terminology) 4-year BS/MS degrees. However, it's still not uncommon to go straight from a bachelor's to a PhD program.
(As an aside, several UK schools are now trying out the American model, where the MS is integrated into the PhD for a 4-5 year program.)
Addendum: MIT encourages students to graduate in 4 years. Or at least, they only guarantee funding for 4 years, though they say that their "tradition has been to support fifth-year students who have made progress on their thesis problem and who need the fifth year to strengthen the result". It's all about the thesis.