Ah, yes the degree conversions. I faced a similar situation as I;m a History Honours graduate from Delhi University, and the highest ever awarded would be somewhere between a 65-67. For subjects like history and english (atleast in the indian system), anything above a 60 is a first division and distinction!
Strange UCL would ask you for a 70, cause i was told most UK colleges recognize the fact that certain humanities subject never go beyond the late 60s.
Anyhow, what i did was - i sent in a transcript from my college that clearly mentioned i had a first division and distinction. In my CV i stated that i had secured the third highest in my graduating class, and also put in something like 'stood 11th out of 350 students from DU' as Delhi Uni provides all results online. Annnnd (if that wasn't enough) my referees all mentioned i was amongst the top students.
Ofcourse - i doubt you'll have to go to those lengths but a letter from your Uni stating that a 70% in english honours is RIDICULOUS (in much politer terms ofcourse
) should be enough.
I figured if they were not familiar with our grading system, drawing constant attention to the fact that i was in the top 5% of my graduating class would be enough to equate my 63% with a first.
Good luck, and hopefully Oxford works out !
Edit: Also, if it helps, i know many a-people who got conditional offers from places like LSE (to which every DU graduate applies) requiring 65%+ on graduating, which ofcourse they couldn't meet. All they did was write to the school detailing how 65% was a bit of a stretch, and i know quuiite a few who then got successful offers.