I like the Gherkin --it's fun, and every city needs a new phallic landmark now and then in the style of its time. You can consider it a success to some extent just by virtue of its having been given a nickname at all. FYI, treehugger has a link to an Economist article about the Gherkin's green features (
The Economist on Green Building and Swiss Re Tower : TreeHugger)
I don't know of any other green buildings in London, but they are getting ambitious about bike paths, which to me is a hallmark of sustainable urban planning:
Streetsblog London Announces Billion Dollar Bike Plan