Understanding you did resign BUT it may be worth contacting Acas helplineTelephone: 0300 123 1100Monday to Friday, 8am to 6pm
The reason I am suggesting this is because even though you resigned, you only did so under "duress", which is an attribute of unfair dismal.
Hopefully you may have records of the "call to chat", those attending, anything that would suggest pressure.
Also how did it become public?
You have the positive feedback documents so this would indicate the business did not see an issue with your work. There was no requirement for disclosure so again supports you.
Going back some time to my HR days but protecting informants when you have done nothing wrong in the work place, is again discrimination. They must have records of the events, you should be able to get them, especially the threat by the person to go public. Somethings may of changed but discrimination has not so what have you got to lose.
I know it is easy to say but you should not be scared as confidentiality is on your side.
You did not say whether an NDA was involved? If so I believe it would hold no grounds in something like this.
Good luck and be brave, just research, phone the hotline for free advice and possibly as a decent employment solicitor not an "ambulance chaser".
Learn from yesterday, live for today, hope is for tomorrow else what is left if you remove a mans hope.