Since 1988, the most comprehensive identification book for British marine gastropods has been Graham, 1988, Prosobranch and Pyramidellid Gastropods. Synopses of the British Fauna (New Series) no.2 second edition (662 pages) by Linnean Soc. [NOT first ed.112pages]. It has long been out of print and difficult and expensive to obtain second hand.
The Linnean Society through the Field Studies Council is publishing a revised edition of Graham (deceased) with Geoff Wigham as revising author. The first two parts of three are now on sale. The links below are to the publisher's sale site (FSC) and include a £5 reduction off part 2 if bought before Sunday 26th November (the delivery charge in British Isles for the two together is £5, same for a single part). Total charge £75. I think this is the most economic offer available, but check online to be sure. You will find the same two books and delivery from NHBS is £102.49. I have no connection or benefit from this publication or publisher other than that the frontispiece in each is a photograph of mine.
Regards, Ian Smith
Wigham, G.D. & Graham, A. 2017. Marine gastropods 1: Patellogastropoda and Vetigastropoda. Synopses of the British Fauna (New Series) no.60. (172 pages). Field Studies Council. Telford.
and
Wigham, G.D. & Graham, A. 2017. Marine gastropods 2: Littorinimorpha and other, unassigned, Caenogastropoda. Synopses of the British Fauna (New Series) no.61. (344 pages). Field Studies Council,Telford, England.