Advertising Standards Authority will include online advertising as a part of it's brief:

http://www.bbc.co.uk/news/business-12435838

"From the start of March, the ASA will be able to police any statement on a company's website which could be interpreted as marketing, even if it is not a paid-for advert. "

Which suggests that if a company makes ridiculous environmental claims, it should be possible to inform the ASA.