Collins Complete British Birds Photoguide has everything you would ever need in a bird book. It gives a very detailed introduction to British birds and a highly useful section on bird habitats, plumage, migration and movements and conservation at the start of the book without making the text sound too boring.
The photographs in the book are absolutely superb and show birds in every plumage; male and female, adult or juvenile, summer or winter and they are very clear. Each species account gives a description of plumage patterns, voice, where the bird is likely to be found and its status in the country at large, and also (which I found very useful) a bit on where the best place to find the bird is and at what time of year, the distribution maps are very clear and include a calendar bar to show if the bird in question is a year round resident or a visitor.
I found the text very easy to read (unlike most bird books which tend to be very scientific and hard to understand) and is a great book to have. Buy it, I guarantee you will not be wasting your money.