The Oxford Book Of Wild Flowers by B.E. Nicholson, S. Ary and M. Gregory was published in 1960 (reprinted with corrections in 1962). 

The plants are usefully arranged by colour, and hand painted by esteemed British botanical artist, Barbara Nicholson.
According to the book, the illustrations were drawn from live specimens, over half of which were collected by the artist herself. The remainder were volunteered by Flower-enthusiasts across the British Isles.

I inherited this book from My Grandfather, who was a keen plant-enthusiast himself.