"The Hamlyn Guide to Edible and Medicinal Plants of Britain and Northern Europe" by Edmund Launert is intended as a field guide, but lacks keys. I'm not entirely sure how useful it might be. Apart from plants, it also includes fungi. Colour plates and text are on facing pages. Essentially, this is a field guide with added tips on how the plants can be used in food preparation or medication. Some of them are unexpected. Did you know that parts of the fireweed can be eaten as a vegetable, and that its leaves are good for tea?
Since fireweed is about the only wild flowering plant I can identify, and since it grows pretty much everywhere, I suppose I could survive being "left behind", after all... :D