MSc vs. BSc admission GPA for an application to a PhD program
Typically, grades in undergraduate work are weighted more heavily than grades in graduate work. This is because your graduate work in general is focused on the research you do and the papers you publish as opposed to merely classwork. However, this is not to say that the answer to your question is clearly black or white. Indeed, any reasonable school will look at all aspects of the applicants (grades in both undergrad and beyond, summer projects, research experience, letters of recommendation, etc) when deciding if they are a good fit for their program. There are no doubt countless examples of students whose grades differed from the other applicants, but were accepted anyway. 

That being said, it's usually seen as a positive if your grades trend upwards near the end of undergraduate work and the beginning of graduate work. The idea is that if an applicant shows recent signs of improving, they (hopefully) are more likely to continue improving after joining the new program.

Finally, it's worth noting that different schools (and even different departments within a single school) handle their admissions process differently. As always with these types of specific questions, it's a good idea to ask the school's department directly about the matter. In particular, I'm sure that there's a contact in the CS department in Oxford you could email who could be able to answer this question directly.