We are planning on holding the 2008 AGM in London.

Can you suggest any suitably green/eco venues?

Not especially green, but with a long history as a place of radical thought and probably around the right size:

The Conway Hall http://www.conwayhall.org.uk/

A beautifully restored building used by the SPAB for meetings (maybe a little small for the Ecology AGM):

The Artworkers Guild http://www.artworkersguild.org/

Both are a walkable distance from Kings Cross and Euston stations, or very close to tube stations.