Which denominations are considered mainstream in the UK? Wikipedia lists these denominations as "mainline protestant" churches in the United States:


American Baptists
Disciples of Christ
Congregationalists / United Church of Christ
Episcopalians
Lutherans
Methodists
Presbyterians


However in the UK some of those denominations don't exist; I've always understood the British mainstream protestant denominations to be:


Anglican
Baptist
Methodist
United Reformed Church (The Congregationalists and Presbyterians merged a few decades ago to form this denomination).


Does such a definitive list exist anywhere? Is there a way in which some denominations are "officially recognised" in the UK while others aren't?
This 2007 report entitled Churchgoing in the UK has a vast array of demographic data on churchgoers in the UK. It even differentiates between regular churchgoers and those who just state affiliation.

For example, among regular churchgoers surveyed, the denominational breakdown is:


Church of England 31%
Roman Catholic 29%
Church of Scotland/Presbyterian 9%
Methodist 6%
Baptist 4%
Pentecostal/New churches 7%
Other 7%
Non-trinitarian 3%


All these are defined in the report (especially Appendix 1 Figure 10). This should answer mainstream as in "most common." Mainline is defined as a specific doctrinal stance in the Wikipedia article you link, so denominations on that list would be "mainline Protestant" there too. Though some don't exist there, I think there aren't any that exist there that aren't here/in that master list.