Originally published in 1989 as an Illustrated Flora, this book has been substantially updated and is unbeatable value when purchased with Cassell's parallel guide to Trees. Christopher Grey-Wilson is the only real competitor to the late, great Francis Rose as an accessible authority on wild plants in Britain and northern Europe. (And again with Marjorie Blamey, he's also behind A&C Black's indispensable Meditteranean wild flower guide.) This guide covers some 2,500 species and Blamey's illustrations are fabulous - each page comes alive. The book contains many naturalised species introduced over the last couple of centuries, providing enlightenment on their origin. The only real criticism of this book is its lack of a key. If you're knew to wild flowers you'll spend a lot of time flicking through looking for the kind of plant you've found. But once you're in the right area, there is a great description and very good additional mini-illustrations of key parts and seeds, and similar species, to help you pin-point your find. On the downside, some of the illustrative details get a little fuzzy here and there, sometimes lacking precision.
I use this book a lot, but it's way too big for a field guide. For many years, I've relied instead on Francis Rose's wild flower key which was the only one in its class, although some of the keys were a bit dodgy. I'm delighted that Rose's 1981 guide has finally been updated by Warne: The Wild Flower Key (Revised Edition) - How to identify wild plants, trees and shrubs in Britain and Ireland. I would recommend that as the ideal choice for enjoying wild plants in Britain.
Another alternative for the naturalist is Fitter, Fitter and Blamey's The Wild Flowers of Britain and Ireland: A New Guide to Our Wild Flowers, published by A&C Black. That too lacks a key, but makes up for it to some extent with a mini-key that heads up certain groups of difficult plants. It also covers a lot of plants that Rose does not, including grasses and ferns, which makes it a better choice for beginners or those looking for a single guide.
I note, however, that Claire O'Reilly, in her review of the new edition of the Rose Key, warns that Fitter and Blamey's 2003 guide contains "many" errors. They published two guides in 2003, both referred to above, so I'm not sure whether the errors are found in just one or both. I'm not expert enough to be sure and, since errors will not help me get better at plant ID, the Rose guide might be the best choice overall.