This subject interests me as well. Here is a short list I obtained form someone who works in the field of sustainable development....
KPMG has a Sustainability Advisory Services group. Check them at http://www.kpmg.co.uk/kpmg/uk/servi
PricewaterhouseCoopers does some of this work as part of their Risk Assurance Services:
Finally, the Corporate Citizenship Company is a London based organisation that does [work on sustainability]: http://www.corporate-citizenship.co.uk