Can you recommend a good conference speaker in UK on the subject of Energy & Sustainability?

Your best bet would be to do a search on the net for organizations in that field and then see if they have a Speaker’s Bureau.

Here is a talk by David McKay on TED

There are numerous speakers out there but I found Charlie Browne is the right person that can make your event successful. He is one of the prominent speaker from jla.co.uk and IKEA UK’s Environmental and Sustainable Development Manager. His style of delivering speech is marvelous.