I have been asked to reply on behalf of the Department for Business, Innovation and Skills.
The Impact Assessment Guidance published in August 2011 states that impact assessments are generally required for all UK Government interventions of a regulatory nature. Regulation is defined to include guidance with statutory force or Government backing, but not voluntary guidance and commitments (page 19 IA Guidance). As such, impact assessments are not required for voluntary guidance and commitments. However, to the extent that voluntary guidance is presented as an alternative to regulation within an impact assessment for a regulatory proposal, this will be captured in those documents. The development of Impact Assessments is the responsibility of individual Departments.