The best British birdsong app for iphone available at the moment is the onomatopoeically-named Chirp! Bird Songs of Britain and Europe
You can listen alphabetically, by song type and commonness, but best for all for me is the quiz which is completely addictive. The commonest British and Northern European birds are featured, with more birds promised soon in the updates.
This is a really great app for learning about birdsongs. Well done to the bright spark who made it. More here from itunes
See also wildflowers app