I have no idea if it's any good compared to others mentioned but I have
Collins Wild Flowers Which is pocket sized (11cm x 8cm) 240 pages and is full of drawn illustrations, Marjorie Blamey & Richard Fitter 350 species. The new edition has photographs aswell apparently. I got mine in a charity shop for 50p.
It really just identifies the plants so you can read further on them later when you get home. It gives one line about the plants history and growing location. For example
Vipers Bugloss
Echium Vulgare A big stand of this bugloss with its brilliant blue flowers is one of the most gorgeous sights in British flora. It grows on dry bare spots on light soils, esp chalk and flowers June - September.
Common Valerian
Valeriana officinalis is a tall plant with pinnate leaves and conspicuous clusters of small pink flowers that appear June - August. It grows in both damp and dry woods and grassland. It's root used to be regarded as a cure for hysteria and other nervous conditions.