I assume kipfilet is referring to Ricardo Reis. I think people should put the bar lower...
With a reasonably good First, or even high 2.1 with good references and firsts in the important courses (like 202 and 221, also 210 if you take it, and real analysis) your chances are pretty good for MPhil Econ at Oxford.
The 80% thing is a ridiculous standard, I had two friends that went from Econ and Maths & Econ to Ox MPhil and though both were very good they didn't have 80s! If you are very good you might consider MRes track 1 at LSE and then apply to the US during your first year. If you stay in the UK, LSE is arguably better (at least broader) than Oxford for a PhD.