Any papers you plan to use in the Research Excellence Framework for 2020 (for those not acquainted, an exercise every few years where UK universities get to say "look how good we are, we published this many papers, please keep funding us") must be available as open access. It isn't necessary for these papers to have gone the gold open access route (where you pay a *!&^%-ton to the publisher), so you could instead upload the non-published version to a preprint server (which is a lot cheaper).