I applied there last year,
1) I had a guesstimate excercise which was to estimate how much revenue the London Eye takes in a year.
And to work out how many newspapers are read in Britain
2) I also had a short oral case study, which was a project they were currently facing related to IT. I'm sorry my memory is a bit hazy on the specifics but I do remember the majority of the interview questions were "what-would-you-do-if-you-were-in-this-situation" type questions and were questions relating to my past work experience.
I actually enjoyed the interview process and got to the offer stage despite not doing particularly well in the cases - so guess they are assessing your poise under pressure, getting an insight into how you think logically ( so think out loud)and also what makes you stand out personality-wise (are you a likeable person) from all the other well-qualified candidates.
But practice does make perfect - try the Vault Case Study guide, and Wetfeet -Ace Your Case series (these are reaaly good and you can order them from Amazon). But if there's no time try reading through Bain's client success stories, as a guide they are structured like case studies - problem, approach, recommendation
Hope this helps and good luck!