As Peter says, lots of stuff in the H&S world but not much that would help you create a policy.  I did once see a sensible webinar on developing a well-being strategy which I think was from the CIPD.  The CIPD no doubt have lots of credible stuff on this.  

Also as Peter says, you need to talk to HR about this.  Some HRs will insist that this is their area and nothing to do with you, others will not want to get involved because all they care about is hiring and firing (in this case, good luck with any well-being project), and there is an outside chance that there are some HRs that would welcome your collaboration on this.

Once upon a time I was in a similar predicament.  I went to a mental health first aid course where the trainer asked us whether our companies had a well-being policy.  I said "We don't, but I could be in a position to write one.  Can you give me any pointers to sources for what to put in it?"  The trainer said they would get back to me, and evidently asked someone about this before coming back with the suggestion of "The Health and Safety at Work Act".  Which of course, is of precisely zero use for this purpose.