London's most exclusive neighborhoods sit on sites of the some of the most sinister and scandalous crimes in British history.   Stories of violent death will always hold us in a grim but thrilling grip. The dreadful crimes related in Foul Deeds in Kensington & Chelsea are shocking examples of murder cases that readers will never forget. Crimes of passion, opportunistic killings, political assassinations--the full spectrum of extreme criminality is recounted here.   John J. Eddleston has selected a series of notorious episodes that give a fascinating insight into criminal acts and the criminal mind. The human dramas he depicts are often played out in the most commonplace of circumstances, but others are so odd as to be stranger than fiction.   Cases involving the killing of wives, lovers, and children are among those he describes, but he also reconstructs in forensic detail several more unusual crimes--two men shot dead at a lecture, the field marshal who was assassinated on his doorstep, the acid bath killings, and the murders of two ill-fated countesses.   These lethal episodes give a fascinating insight into the dark side of the history of Kensington and Chelsea.