I think KPMG's case interview is very different as compared to traditional strategy consulting firms. I applied 2 years ago and got the offer. They would give you a topic, ask you to prepare for it for about 5 days and come back for a 10-minute presentation. Then, it's Q&A both on the topic and also competency-based questions.
Preparing strategy cases would help with your communication and commercial awareness, but generally it does not directly contribute to your performance at a Big 4 interview.
P.S. I applied to London 2 years ago - not sure if they have changed the structure now.