The Field Studies Council has recently published "The conifers of Britain, an identification guide" More...

This takes the form of a comprehensive set of species accounts and two different online interactive keys, a single column key which displays a lot of information on how each of the species 'scores' against various character, and a direct comparison tool which helps you to make a final decision if you're still unsure (useful!). 

The online keys can be accessed at:  https://conifers.fscbiodiversity.uk