HEFCE, on behalf of the four UK funding bodies (including HEFCW), has recently updated its policy for open access in the next REF (2021). It updates and supersedes the previous policy for open access in the post-2014 REF (March 2014), which was later updated after feedback from the sector. The only substantive change is an alteration to the timetable –  the requirement that outputs should be deposited on acceptance will now start after 1st April 2018. For further details, see links below:

2016/35 Policy for open access in the next Research Excellence Framework: Updated November 2016.

http://www.hefce.ac.uk/pubs/year/2016/201635/

Circular letter 32/2016, Open access in the Research Excellence Framework: Extension of flexibility.

http://www.hefce.ac.uk/pubs/year/2016/CL,322016/