Is a degree from Oxbridge more difficult to obtain? I am not convinced it is more difficult.
It may be more intellectually difficult. But considering you have far stronger support mechanisms at Oxbridge, including tutorials that are often one-on-one or in very small groups, it could be argued it is easier to obtain a strong grade in an Oxbridge degree because of the mechanisms in place to get you there. Constant, regular and detailed feedback from your first week undoubtedly allow you to learn more, quicker.
So maybe the student doing the same course at Reading or Oxford Brookes has to work harder to achieve the same grade where they have to be more self sufficient? And that could be why it is more difficult.
Just my opinion on the matter anyway.