I've been using Blamey, Fitter and Fitter in its various disguises since 1976. It's currently called "Wild Flowers of Britain and Ireland", and my version was published in 2003 by A&C Black, ISBN 0713659440. It's well-illustrated, well organised (apart from the grasses section), handy enough for the field, and I recommend it very highly.
Anything more technical and I use Stace, but this isn't so easy to stuff into a rucksack and requires a degree of technical knowledge before use.
Hope this helps. Rgds, Tim H