The new HEFCE policy is now in effect. Journal articles and conference papers (published in conference proceedings with an International Standard Serial Number) accepted for publication after 1st April 2016 must be made open access in order to be eligible for submission to the post-2014 REF. There are two ways in which you can do this: via the Green route or via the Gold route.
Green Open Access
Green open access means publishing in a subscription journal and depositing the final accepted version (post-print) in an institutional repository, such as Pure. To be compliant with the Open access policy for REF 2021, researchers need to do this within 3 months of acceptance. The Library Open Access Team will make the paper open access on the Research Portal, usually after an embargo period. The advantage to this route is that the publisher requires no payment.
Gold Open Access
Gold open access means that the published version of the research paper is freely available online in an open access journal. There is no embargo period, so access to the full article is immediate upon publication. Journals often require the researcher to pay an Article Processing Charge (APC) and this cost can be prohibitive. However, the library can potentially help with the payment of APCs, via various funds and membership schemes. This route is compliant with most funders, including HEFCE (it is seen as a viable exception to the policy).
As you can see, there are benefits to both options. The most important thing is to make your work open access.