The definitive plant ID books are "The Wild Flowers Key" by Francis Rose and "Wild Flowers of Britain and Ireland" by Marjorie Blamey et al. However, both are botanical illustrations not photos, but they are the two books used by botanists. Rose uses a key which takes some getting used to, but is excellent.
I have just bought "Harrap's Wild Flowers" by Simon Harrap - good clear photos but limited to 934 species (Rose is 1,600 species).
You might also consider Geoff Dann's book "Edible Plants" if it's more foraging your interested in - good clear photos of over 400 species (Geoff sometimes contributes to threads on this forum).