Trees for Cities
Trees for Cities is an independent charity set up in 1993 to work with local communities on tree planting projects. Their aim is to tackle global warming, create social cohesion and smarten cities through tree planting, community, education and training initiatives in urban areas.
http://www.treesforcities.org/html/aboutus/
WWF
WWF is the world's largest and most experienced independent conservation organisation. Much of their work in the UK is directed global programmes but also seeks to influence global environmental issues through responsible actions in the UK.
http://www.wwf.org.uk/core/about/whatwedo.asp
Woodland Trust
The Woodland Trust is the UK's leading conservation charity dedicated to the protection of our native woodland heritage. The trust acquires woodland sites and brings them into there care and protection. They also replace those woods that have been lost to landscape and create more new native woodland than practically anyone else in the UK.
http://www.woodland-trust.org.uk/findoutmore/index.htm