Econ0, MPhil is, of course, not the same as PhD, but you can continue to PhD (or DPhil as they call it) if you pass the core exams with a score above 63%.
What do you mean when you say that they don't have a PhD program? Of course they do, it's just that it is called DPhil instead of PhD.
Also, what are the additional options Cambridge has and Oxford doesn't?