Sorry, but I feel pedantic today. Anyway:
1) 'Which is the best at economics' - how is that a question? Do you mean which has the best reputation? Or which is the hardest to get into? And so on. You can't ask which is 'best' because it has no meaning - how do you define a university being 'best' at something? All 3 of the universities are excellent for economics and their quality at it shouldn't be the deciding factor between the 3. Instead look at the accommodation, social life and so on. Where would you most like to study for 3 years?
2) If you think you are up to the standards of Cambridge of course it is realistic to apply there. Your GCSE's and maths A-level sounds ok, but we need to know your A-level subjects - for example if you are taking subjects like business studies, accounting and media studies, it makes it less realistic to apply to any of them.
3) 'Their course looks very appealing but the competition appears stiff , would Oxford be a better bet?' - Very annoying question. Both are extremely competitive - everyone applying will probably be AAAA. Same is true for LSE. Thus to think that because Cambridge has a lot of competition you'd have a better chance at Oxford is ridiculous. In fact if I recall correctly, Oxford's econ+man course has more applicants per place than Cambridge's economics course.
4) I'm not going to give you tips on preparing for interviews, but looking at the recommended texts and reading a broadsheet newspaper is a good start. Look around on this forum and you'll probably be able to find stuff to help you.
5) Interview format (I only know about Cambridge)
Check out
this page to see how individual colleges interview. Some use the thinking skills assessment, others use written work. It varies from college to college.
Here's an example of how one college does it:
- 2 interviews
- First is about a piece concerning economics (such as a newspaper article) that is given to you to look at before you go into the interview. You need to explore the issues raised by the piece.
- Second is a general interview, with the questions coming from what you say in your personal statement. If you say that you're interested in the issues raised by globalisation, expect to be asked about it.
6) Look at the Alternative Economics League Table at the top of this forum. It lists the universities by how competitive the economics courses are. Even if you do want to apply to Oxbridge/LSE you'll need to look at other universities as you get 5 choices and economics is one of the most competitive courses to get on at degree level. People with amazing grades and personal statements have been rejected due to the sheer competition.
Hope that helps a little and sorry for taking apart your original post!