The Keswick diet (also called Fat Fast diet) is just another fad diet.
It was developed by Alan Kekwick and Gaston Pawan. The plan restricts participants to 1,000 daily calories, with 90 percent of these calories provided by fat sources.
People loose weight on the diet because it is a [FONT=Museo Sans 300, Arial, sans-serif]starvation[/FONT] diet. It is nothing magic. With how low the calories are on the diet, and how little protein is included in the diet, it is a pretty sure way to loose muscle.
The best diet to loose body fat while holding onto all your muscle mass (or even gaining muscle mass) is carb cycling. Here is an article I wrote that explains how to set up a carb cycling diet for fat loss with a full sample meal plan: http://www.evolutionary.org/cycling-carbohydrates