All the environment statements, impact case studies and impact templates from REF2014 are available on the REF2014 site. This is an incredible source of best practice material.
If you have not already done so go to the REF 2014 site, look for institutions in your UOA that you feel are in some way comparable to your own … especially those that scored well in impact or environment. Download their environment statements and impact templates and study them.
Please, please, do not do this in order to think “ah they did this we could copy that to look good on REF submission next time”.
However do do this in order to think, “that looks a good idea, that would work here”.
That is use them as a best practice guide to find ways to encourage and grow research and impact in ways that make sense in your own institution, department or research group.
Of course as it comes to the time to write your REF2021 submission you can also use them to remind yourself of things you do that are worth reporting.