Collins Tree Guide Comprehensive, detailed and well-laid out. An excellent field guide.
Trees in Britain, Europe and North America Despite its age, this remains an excellent reference. I find it particularly useful for coniferous trees. Photos throughout.
The Wild Flowers of the British Isles The BEST visual guide to the wild flowers of Britain. The illustrations are superb. Unfortunately it's out of print. Grab a second-hand copy while you can!
The Wild Flower Key (Revised Edition) - How to identify wild plants, trees and shrubs in Britain and Ireland A good detailed field guide. The illustrations aren't to the standard of Garrard & Streeter but the key is is powerful for identifying unknown plants.
British Wild Flowers: A photographic guide to every common species (Collins Complete Guide) A good photographic guide with some useful plant descriptions that are lacking in other books. Plastic cover good in the field.
Grasses, Ferns, Mosses and Lichens of Great Britain and Ireland I find this particularly useful for ferns and mosses. Photos throughout.
Grasses, Sedges, Rushes and Ferns of Britain and Northern Europe - Collins Pocket Guide. With clear illustrations throughout, this guide is particularly good for grasses, sedges and rushes, an area of plant life that can be baffling.
Mushrooms An updated classic. Very useful guide with over 1,250 photographs.
British Trees: A photographic guide to every common species (Collins Complete Guide) Nice photographic guide to British trees.