You will find ( I think the answer is yes ) that it is a power ( energy companies can offer it if they want ) and not a duty ( you can make the energy companies dance to your tune )
yeah I have checked - you have entered this under 'Law'
and they MAY have a list you can choose from - which of course doesnt include your fave charity.
( you would be better off - I mean the charity better off - doing thro GiftAid which has some wowee ! tax reliefs attached )