There has been some disquiet indicated concerning the wording of the original petition linked to in the previous posting which has prompted a certain amount of debate over at Leiter Reports. There is also a serious discussion being conducted concerning the notion of "impact" assessment over at Crooked Timber.
By contrast the Arts and Humanities Research Council contains a number of pages on its site that discuss ways of applying the notion of impact within the area of humanities research which you can find here. The basic justification of inclusion of the notion of impact is also set out by the Higher Education Funding Council for England on its site here.
The question also arises as to whether the opposition parties here in the UK have different views on this from the present governing Labour Party who, in their website statement of policies on education, make no mention of the Research Excellence Framework or the policy on "impact".