The AIDGAP guides, or Aid to Identification in Difficult Groups of Animals and Plants, are produced by the Field Studies Council specifically for use by non-specialists. They are often the only accessible key available for some taxa.
I do not own many of the available titles, those I possess are shown above. Sadly, I did have a couple more but again, lent to ‘friends’ they have not returned. The keys in these publications have been thoroughly tested by sixth-form students and above, as a consequence they are invariably quite easy to use. If you have only the slightest interest in one of the many little groups that these publications cover, then treat yourself to a copy as they are quite inexpensive.
Be aware, these guides seldom go down to species level, being more aimed at the higher level, taxa etc. If you are a specialist or want to go further down the identification path, then one of the volumes- Synopses of British Fauna, may suit you better.
Books from my shelves:
A key to the major groups of Freshwater Invertebrates, 181, P.S. Croft. Field Studies Council. Reprinted from Field Studies, Vol. 6 No. 3, 1986. ISBN 1 85153 181 6
A key to the Families of British Beetles, 166, D.M. Unwin. Field Studies Council. Reprinted from Field Studies Vol. 6 No. 1 1984. ISBN 1 85153 166 X
A key to the major groups of British Terrestrial Invertebrates, 187, S.M. Tilling. Field Studies Council. Reprinted from Field Studies Vol. 6 No. 4 1987. ISBN 1 85153 187 5
A key to the Woodlice of Britain and Ireland, 204, Stephen Hopkin. Reprinted from Field Studies Vol. 7 No. 4 1991, ISBN 1 85153 204 8
Keys to the families of British Spiders, 197, L.M. Jones-Walters. Reprinted from Field Studies Vol. 7 No. 2 1989, ISBN 1 85153 197 1
A key to the Families of British Diptera, 143, D.M. Unwin. Reprinted from Field Studies Vol. 5 No. 3 1981, third reprint 1992.
Bees, Ants and Wasps, a key to genera of the British Aculeates, 7, Pat Willmer. Field Studies Council, 1993, first published 1985. ISBN 1 85153 807 1
A key to the major groups of British marine invertebrates, John Crothers. Reprinted from Field Studies Vol. 9 No. 1 1997. ISBN 1 85153 200 5