The 2014 Atlas maps the distribution of S. capillifolium and S. rubellum together as the 2 species were separated only later on and bryologists didn’t consistently record the separate varieties prior to this.
View distribution from the BBS Atlas 2014There are several publications and keys dedicated to the Sphagna of Britain and Europe. The most recent of these are listed on the Bryophyte identification page under Resources.
Bryophyte identification resources