The identification of British (and Irish) Ephemeroptera is covered by two scientific publications by the Freshwater Biological Association (FBA).
Elliott, J.M. and Humpesch, U.H (1983): A key to the Adults of the British Ephemeroptera with notes on their ecology. Scientific Publications of the Freshwater Biological Association, No. 47, 101pp.
Elliott, J.M., Humpesch, U.H. and Macan, T.T. (1988): Larvae of the British Ephemeroptera: a key with ecological notes. Scientific Publications of the Freshwater Biological Association, No. 49, 145pp.
These taxonomic keys provide the information required to successfully identify most of the British Ephemeroptera. They also provide extensive notes on their life cycles and ecology.
There is also a key to the Ephemeroptera published in the Naturalist Handbook series:
Harker, J. (1989): Mayflies. Naturalists' Handbook, No. 13, 56pp.
It should be noted that there have been some recent revisions and additions to the British Ephemeroptera that are not noted in either the FBA keys or the Naturalists' Handbook. Full details of these revisions and additions can downloaded from the download area.