If you are looking for nature in London, an organic lettuce is a good place to start - at least, it was for me today. We've hit the Spring equinox, nature is bursting forth and so are blogs. Or, at least, one particular Daily Photo Blog - London Daily Nature Photo. Not satisfied with his prime blog, Planarchy, Nic (a regular comentor here) has started this one. I have to tell you, his photos are really quite good and likely to be well worth the entry price (your time).
I also love the idea of meeting the bits of London's nature scene I mostly tend to pass by.