why was the above project started?
It helped initiate the effort to build the first atomic bombs in the United States during World War II, and helped carry it through to completion in August 1945 by supplying crucial expertise.
Introduction
helping initiate the effort to build the first atomic bombs in the United States during World War II, and helped carry it through to completion in August 1945 by supplying crucial expertise. 
who headed up the manhattan project in britain James Chadwick who is he? He was a British physicist. what was he known for? His discovery of the neutron. can you describe this thing he discovered? It is a subatomic particle, symbol or which has a neutral (not positive or negative) charge and a mass slightly greater than that of a proton.
Particularly alarmed by the notion of a German nuclear weapon project. Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was a research and development undertaking during World War II that produced the first nuclear weapons.
British contribution to the Manhattan Project Manhattan Project Manhattan Project
Origins Introduction Introduction
Refugees from Nazi Germany and other fascist countries were particularly alarmed by the notion of a German nuclear weapon project.  The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom (which initiated the original Tube Alloys project) and Canada. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the U.S. Army Corps of Engineers. Nuclear physicist Robert Oppenheimer was the director of the Los Alamos Laboratory that designed the actual bombs. As engineer districts by convention carried the name of the city where they were located, the Army component of the project was designated the Manhattan District; "Manhattan" gradually superseded the official codename, Development of Substitute Materials, for the entire project. Along the way, the project absorbed its earlier British counterpart, Tube Alloys. The Manhattan Project began modestly in 1939, but grew to employ more than 130,000 people and cost nearly US$2 billion (equivalent to about  The Manhattan Project was a research and development undertaking during World War II that produced the first nuclear weapons. It was led by the United States with the support of the United Kingdom (which initiated the original Tube Alloys project) and Canada.
wiki:7566710
British contribution to the Manhattan Project [SEP] Introduction
Britain contributed to the Manhattan Project by helping initiate the effort to build the first atomic bombs in the United States during World War II, and helped carry it through to completion in August 1945 by supplying crucial expertise. Following the discovery of nuclear fission in uranium, scientists Rudolf Peierls and Otto Frisch at the University of Birmingham calculated, in March 1940, that the critical mass of a metallic sphere of pure uranium-235 was as little as , and would explode with the power of thousands of tons of dynamite. The Frisch–Peierls memorandum prompted Britain to create an atomic bomb project, known as Tube Alloys.