For English Methodists, Methodist Union refers to the joining together, in 1932, of several of the larger Methodist denominations. These were the Wesleyan Methodists, the Primitive Methodists, and the United Methodists.
Various groups, which had separated from the Wesleyan Methodists since the death of John Wesley, decided to join the parent body. After 1932, the new body was known simply as The Methodist Church. To distinguish this from Methodism in other countries (chiefly the USA), it is now styled the Methodist Church of Great Britain.
The various Methodist denominations
The largest was the parent body, the Wesleyan Methodist Connexion, from which a number of offshoots had sprung.
The Primitive Methodists were the second largest of these, having arisen in the first decade of the nineteenth century following the conversion of Hugh Bourne and a number of others in Staffordshire to the north of Stoke on Trent. They had sought to recover the early faith and practice of John Wesley at a time when the Wesleyans were hoping to become more respectable. Their return to Wesley's field preaching, notably in the form of Camp Meetings, did not suit the Wesleyans at that time, and Bourne was put out of membership along with several of his companions. Continuing their evangelism, they began the new group, a Connexion in Methodist terminology, in 1811, taking the name Primitive Methodists in 1812.
The United Methodists were the other group involved in the 1932 union. They were the result of a union in 1907 of a number of three smaller groups. These were the Methodist New Connexion (founded in 1797), the Bible Christians, and the United Methodist Free Churches.
As a part of the Methodist Union, a new volume, The Methodist Hymn Book, was compiled and published in 1933. This included 984 hymns drawn from the various Methodist groups, as well as a selection of the Psalms.
Wesleyan Reform Union
The Wesleyan Reform Union remains fiercely independent, and many of its members do not accept the description Methodist. They arose in the mid-nineteenth century, mainly in the Erewash Valley area of Derbyshire, and emphasise the early teachings of John Wesley. They also use a Methodist hymn book. One main difference from other Methodists is that every local congregation is autonomous, and cannot be dictated to by a central body. However, they are joined in a union for mutual support.
Wikipedia article the Methodist Church of Great Britain