Arthur, I’m sorry if this is not what you want to hear but I am struggling to make sense of the principle of your claim.
Whether your employer acted in such a manner that you could not reasonably be expected to continue in their employ is the issue.
The fact that the matter wasn’t dealt with until after you left, in this case, is irrelevant.
Why?
Your grievance was not upheld - which, by not challenging, you effectively accept that you didn’t have one or couldn’t prove that you had.
It follows, therefore, that you cannot prove that employer behaviour materially influenced your decision to leave before your employer’s determination.