What is slang? You are in a London pub and overhear this exchange between the barman and a customer: "There you go, squire" -"Cheers, mate". That is slang: very informal words and phrases used normally only in speech between people of the same social class, age or work group.
This book will certanly help you to understand the language you hear spoken in the street and at times on television and even see printed in the more popular British newspapers.