TL/DR: We don't know yet, but outputs will probably not be portable.
In July 2016 Lord Stern published the Independent Report: Research Excellence Framework Review. The report identified recruitment (i.e., poaching) as a behavior designed to "game" the system and:
We therefore recommend that outputs should be submitted only by the institution where the output was demonstrably generated. If individuals transfer between institutions (including from overseas) during the REF period, their works should be allocated to the HEI where they were based when the work was accepted for publication.
This report was a reflection on the 2014 REF (and earlier RAEs). The post 2014 REF has not been fully defined (or really even partially defined). The Department for Education responded on 13 October 2016 to Lord Stern's report. In general, the response was positive and the Department for Education has asked HFECE to create "specific and detailed practical arrangements". The HEFCE posted a brief on 16 Oct 2016 about the plans going forward. Specifically, they are now looking at "non-portability of outputs". Consultation is slated to begin November 2016.