In March 2014, the Higher Education Funding Council for England (HEFCE), along with the other UK funding councils, announced their policy around open access to publications for REF 2021.
The policy can be found here: https://www.ukri.org/funding/information-for-award-holders/open-access/ref-2021-open-access-policy/.
In summary, this requires all journal articles and conference proceedings accepted for publication from April 2016 to be made open access by being uploaded to a repository within 3 months of acceptance.
The text should be deposited as the author’s accepted manuscript version, which incorporates any changes resulting from peer review. There may be an embargo period defined by the publisher before the text can be made publicly available, but this should not stop authors from depositing their articles within 3 months of acceptance. There are maximum permitted embargo periods, which are broadly 12 months for science, medicine and maths, and 24 months for arts, humanities and social sciences. If the embargo period of your chosen journal is longer than this, please contact the open access team in RIN for advice.
Note that even if the article/proceeding is made open access immediately from the journal website, a copy of the text must still be deposited in a repository. Other types of research output, such as book chapters, edited collections and monographs are not required to be open access in order to be eligible (though researchers are encouraged to investigate their open access options in these cases).
Research England has produced an extensive and expanding set of FAQs on the policy, which can be found here: http://www.hefce.ac.uk/rsrch/oa/FAQ/.