My interview preparation was split into two parts really. The first part was looking at ULC's and Oxfords prospectuses and websites, to try and get as much information about the course and university as possible. The second part was looking through the economics at Oxbridge booklet and thinking up answers to some of the questions listed there.
UCL had sent me a couple of booklets about economics and statistics there, so I used this to glean lots of information from. I tore the relevant pages out of the booklets, and then highlighted the most important sentences. I also collected information from UCL's website, and pieced it together in a word document. I then used this information to prepare myself for questions like which part of the course might I be interested in? And for specific questions for me to ask at the end of the interview. Oxford didn't send me anything like this, but there was lots of information I could use on their website for the same thing.
When I had finished I had a few sheets of paper, printed with details of course at Oxford and UCL, with question I might ask and answers to ones I thought I would be asked. However I was still struggling with the why did you decide to apply here question. In the end I though I wouldn't worry about it too much, and just say that I had looked in league tables and things and it had seemed like the best choice for my course.
A few days before my interview I did some extra preparation by buying magazines like the Economist and Time and reading broadsheet newspapers, so that I could answer questions on current affairs.