A new open access policy for future research assessments, post REF 2014 has been announced by HEFCE and the other three UK funding bodies. The requirements apply to journal articles and conference proceedings which are accepted for publication after 1st April 2016, and do not apply to data, monographs, other long-form publications, creative or non-text outputs.
The new requirements are that peer-reviewed papers must be added to an institutional or subject repository, such as STORRE when they are accepted for publication. They must be discoverable by search engines and downloadable and accessible to all once any embargo has passed. There are limited exceptions to the policy. You can read more about this policy on the HEFCE website.