Yes you are correct in believing that the exams are harder at oxbridge than at other univeristies. However, this is because the course is structured so that they essentially cram in a Masters into 3 years (you can actually ask for a Masters certificate about 3 years after you've left the uni). So a 1.1 at oxbridge would definitely be harder than a 1.1 at, say, cardiff.
As for direct comparisons, it's always very hard to make. But i'd guess it's something like a 2.1 at oxbridge is like a very good 1.1 at other unis. Alternatively a very good 2.2 at oxbridge would probably be a very high 2.1 at another uni. Obviously don't hold me to that. All i can confirm is that the exams are harder....but that's cos people work real hard at oxbridge.