My suggestions would be to obviously get the best grades possible in your GCSEs and AS levels. Take A-level Maths, I know it is not a definite requirement, but it is immensely useful for economics. Two of my friends were admitted without it, but they had to put in a lot more effort in their first year with extra maths lessons. It also helps for logic in philosophy. Start reading (and understanding) relevant books about the subject. It will give you a wider pool of ideas to draw from at interview and it’s worth your while listing some of them on your personal statement.

Some random suggestions would be:

On Liberty, JS Mill

Problems of Philosophy, Bertrand Russell (very short and also on line at http://books.google.co.uk/books?id=HS8KgQCgwEYC&dq=russell+introduction+philosophy&pg=PR7&ots=XYRqjaLIJZ&sig=35X9zD8aY8DafTEmpnrWJW6dE20&prev=http://www.google.co.uk/search%3Fhl%3Den%26q%3Drussell%2Bintroduction%2Bphilosophy%26btnG%3DGoogle%2BSearch%26meta%3D&sa=X&oi=print&ct=result&cd=2)

Utilitarianism, JS Mill

The Republic Plato

Meditations Descartes

For economics I know plenty of folks will suggest Adam Smith’s The Wealth of Nation, but it is a bit of a cliche when it comes to applications and economics has come a long way since then. I would say instead that it would be a good idea to familirise yourself with basic concepts in Game Theory, Microeconomics (eg. markets, supply and demand equilibrium) and Macroeconomics (interest rates, inflation, employment, currencies). Keep up to date with current affairs so read a decent (i.e. not a tabloid) newspaper regularly. The FT is actually very good for learning some basic ideas. The Economist is also a very good read as it gives an excellent analysis of current affairs in addition to reporting. Prominent issues like the Euro, the Stern Report and climate change, constitutional reform, global security, etc, may very come up at interview.