I'm a math undergraduate and I'm about to finish my second year at UCL. It's time to consider my graduate school, and I have some questions about it. In the UK, many universities provide math PhD directly after 3-year-undergrad, and they provide MSc for undergraduates as well.

Take Oxford as an example. It says that you don't need to have any publication before the doctoral programme and that you can apply it with a First-honour degree. But I wonder is it really possible to apply for a PhD without a master degree? Personally, I view MSc as a preparation course for a PhD. Therefore I want to know, is MSc a prerequisite for PhD, and if not, how it is possible that a 3-year student is able to do research with other 4-year-students?