It's pretty obvious that in general Oxford students are brighter, work under more pressure and get more firsts than people at other universities. They are, after all, completing their degree in a much shorter time span. What is not so obvious is the question of the standard of the degree obtained. Is it, for example, easier to get a first from Cardiff than Oxford? I would think that in certain subjects eg mathematics, it would be pretty easy to compare final exams and see if they are the same difficulty - most uni's have their past papers in the library. Universities have external moderators to make sure that their standards do not slip and are comparable to others, so I would think that a first is a first is a first.
I'm sure that if there was a huge discrepancy in standards it would undoubtedly have been highlighted on this forum.