The new Lee Valley Whitewater Centre (the Olympic venue for canoe slalom in 2012) is now open for recreational paddling: http://www.leevalleypark.org.uk/en/content/news/white_water_thrills_/white_water_thrills_.aspx
For info about what you can do there, see here: http://www.leevalleypark.org.uk/en/content/cms/White_Water_Canoe_Ce/Activities/Activities.aspx