I got rejected for undergrad studies there despite having near perfect grades, extra curriculars etc - so I''ll try to not let my bitterness prevent mefrom being objective about you're chances lol.
I'm no expert, but if you're planning on doing a postgraduate course in something maths/econ related, it's incredibly competitive. Oxford won't be short of people with very high marks in their degrees who can also self-fund. So no, I don't think you being able to self-fund is going to put you at any advantage. Besides, Oxford admit people based upon merit and the perceived talent of the candidate (they aren't desperate for money, they have plenty of funding - admitting lot's of students reduces the courses prestige/exclusivity). I'm not saying you have no chance, but it will certainly be v.difficult for you with you're GPA.
If you get rejected, you could always do a masters at another institution, and if you do really well, apply to do a PhD at Oxford - this way you'll be spending three years in Oxford's vibrant atmosphere as opposed to just one.Also, I really don't think you should place the brand name of a university as the deciding factor as to whether you want to study there.Both my parents and grandparents went to Oxford uni and I grew up in Oxford - it's a nice place to live and an excellent institution to study at,but what it's actually like to study there is dramatized massively - I mean you only have to interact with the students for this to become obvious.
Take into consideration the field you want to specialize in and if there are suitable lecturers who will be well-equipped to supervise you. For someone who wants to specialize in microeconomics for example, they would be daft to choose Oxford over a place like UCL - which is the best institution in the world for that particular sub-discipline. I hope you get my jist. Good luck!