Compulsory textbook:
Norman Lowe, Mastering Modern British History (4th edition, 2009)
This book will provide you with the essential historical background for the course. Multiple copies are available at the university library. Everyone should read chapters 25 to 30 (stop at the end of 30.4) and chapters 33-34.
Additional reading:
David Reynolds, Britannia overruled : British Policy and World Power in the Twentieth Century (2000)
Chapters 1, 5, 6 and 7 provide further detail of Britain's status in the world over the key period 1919-1960.