Well there is information in the HMRC website and I guess it's knowing where to look. See here:
http://www.hmrc.gov.uk/businesses/giving/gifts-in-kind.htm
It seems to come under the category of gifts of stock to a charity but it doesn't say too much about the practical side of accounting for it.
Like much in self-employed / limited company taxation, there are some clear guidelines, and other areas where things are less clear. Provided you make reasonable valuation judgements about what you are giving away and don't try to abuse the system, it seems OK to do this. I suppose you really want a simple receipt from the charity for potential audit purposes by HMRC.