There's plenty of useful information about composting toilets on The Little House Company site. Here's a snippet referencing changes in legislation in 2010:
The regulations also state that the compost toilets can be connected to an energy source (electricity supply) only for the purposes of ventilation (ie powering a fan) or to sustain the composting process. This would imply that a toilet that heats the contents to dry it out, or incinerates the contents, does not meet building regulations in the UK. We’ve long held the view that toilets which dry or incinerate can hardly be considered ‘environmentally friendly’ due to the amount of energy they consume.And there's a much longer piece going into very useful detail on their site here about regulations concerning disposal of waste from compost toilets.