The British Naturalists Association
If you are interested in the Natural World, and British Wildlife in particular, you can't afford NOT to be involved in the BNA.
This is one of the best respected, oldest and most active organisations concerned with furthering the knowledge of British Natural History in the world.
It is a superb forum for exchanging ideas and discovering what new discoveries have been made as well as encouraging younger people to get the wild bug.
Simon is Vice President of the British Naturalists' Association.