Because students coming for interview have such a diverse range of backgrounds, it is pretty difficult for tutors to judge them based on what they know. You might think you have enough knowledge to pass PPE Prelims, but the tutor won't necessarily care - they'll want to see that you think in the right way so that when new and big challenges come along, you'll be able to work through them without their help.
What this means is that although it is important to read a lot and to keep up to date with current affairs, what you should be doing is not necessarily be learning everything by rote, but rather carefully looking at how people construct arguments, analyzing why certain things happen, and working through the implications of events and solutions / responses.
For this reason, make sure you pay careful attention to newspaper editorials (a good source of argument) and also that you fully understand the principles of an argument when you read it - be prepared to go back and read it two or three time if needs be.
As ever, quality, not quantity is the key.
