Wheels for wellbeing are a local charity (they run sessions at Herne Hill velo) that provide cycling for people with a wide range of disability/impairments.
They're currently bidding for lottery funding which they rely heavily on for survival (they nearly closed down in 2010).
Have a look at the video below - if you want to support them all you need to do is Vote for 'em and vote for them.
My sister is disabled, and I know how much charities like this can help people. She did horse riding for many years locally and this helped her so much.