That latter part is true, but during his lifetime he did organize the Methodist “societies” and eventually registered their meeting-places as if they were nonconformist churches (because otherwise their activities would be illegal). The Methodists were careful to meet at times that did not conflict with the services of the Church of England (actually they pioneered evening services using the new technology of gas lighting), and Wesley strongly encouraged his followers to attend their parish church as well as the meetings of the Methodist “society.” In fact, this practice continued for some time after Wesley’s death–only gradually did English Methodists come to see themselves as a wholly independent church.
However, the situation was very different in America. There Anglicanism had largely collapsed after the Revolution, and had never reached large areas. In fact many people at this time were completely unchurched (one of the great myths of American history is that 18th-century America was a devout, church-going society and has become more secular with the passage of time). So Wesley decided to sanction the establishment of an independent “Methodist Episcopal Church” in America.
From the beginning, though, not all American Methodists were comfortable under this umbrella. First of all, black Methodists were discriminated against and formed two separate denominations (African Methodist Episcopal and African Methodist Episcopal, Zion) with the blessing of the white denomination. (A third black Methodist denomination, the Colored [later changed to “Christian”] Methodist Episcopal Church, was formed in the South after the Civil War.)
In the 19th century, several more groups split away from the Methodist Episcopal Church, mostly because they thought it was too authoritarian (many objected to bishops, though not all did) and did not allow enough freedom both in organization and in worship. They also thought that Methodists were becoming too worldly and respectable and were watering down the doctrine of holiness. These groups tended to be more socially radical–they were more open to the ordination of women and they were outspoken opponents of slavery. The Wesleyan Church is, as far as I know, the oldest such denomination still existing (it was founded in 1843, and merged in 1966 with the Pilgrim Holiness Church, a smaller “holiness” denomination founded in the early 20th century).
The MEC also split into Northern and Southern factions in 1844–as you’d expect, this split was caused by the fact that the MEC as a whole was opposed to slavery, though not as completely abolitionist as the Wesleyans and other splitoff groups.
The northern and southern groups reunited in 1939, also uniting with one of the 19th-century splits (Methodist Protestants). Some of the Southerners formed a new denomination instead, the Southern Methodist Church (a few of the Methodist Protestants also stayed out and as far as I can tell are independent fundamentalists today). In 1968, the Methodist Church (i.e., the church formed by the union of 1939) merged in turn with the Evangelical United Brethren, itself the merger of two German-speaking denominations historically related to the Methodists (again, there are smaller groups from this tradition that remain independent–the college where I teach is run by one such denomination, the United Brethren in Christ). This new denomination was called the United Methodist Church. Both in terms of size and of historic continuity, it can claim to be the primary representative of Methodism in the United States, although the AME and AMEZ would both dispute this.
The short version of this long story is that in the U.S. the United Methodist Church is the largest and most historically central Methodist denomination (at least of the “white churches”), but there are a number of other historically Methodist denominations, of which the Wesleyan Church is one of the oldest.
In Britain, on the other hand, where there were also a number of Methodist splits, the “Wesleyan Methodists” were the largest and most mainstream group (like the “Methodist Episcopal Church” in the U.S.). Other denominations included the New Connexion, the Bible Christians, etc. Most of the Methodist churches in Britain reunited about the same time the Americans did, and this reunited denomination is simply called the “Methodist Church.”
So “Wesleyan” means something very different in Britain than in the U.S., though I believe there is a “Wesleyan Holiness” denomination active in Britain these days.