Seems like you handed them all the cards, and they duly played them.
Mind you, if they have nothing written, dated, and signed from you in terms of resignation then you didn't resign. Walk right back in there if you feel you need to fight this out.
If they didn't offer you to sign the notes etc of your disciplinary hearing then they are null and totally void. There's nothing to say that they have been tampered with.
I'm no lawyer, but I'd expect them to run into difficulty punishing him for not following a company policy that nobody in the company follows. Certainly if he is singled out for not following it.
The same happened to me at my old company, Litterally nobody followed a 'mystical policy' and I was singled out, even if you provide hard evidence everybody else was doing the same.... It works very well, untill you take it to employment tribunal, at that point, they keel over.
This.
Also, a lesson to everyone about why they should be in their union from the start. Unions are amazing for workers. I imagine he would have been completely let off had he been in the union. HR do not work for the employee, they are there to make sure the company does not get in trouble and are equally as out to get you as any vampire capitalist.
Personally, my union were absolutely useless and corrupt, haven't joined one since! Usdaw IIRC.
OP, the worst thing you did, as you now know! (hindsight is a beautiful thing) was resign, and offer to give them money. If you'd played it out longer, maybe even file a grievance, it stretches it out. meaning, they are paying you for longer

This buys you time to seek professional advice etc.
If I was you, and everything you have told us is indeed true, and you really believe you haven't done anything wrong, then you should take it to employment tribunal for sure.
I sympathise though, this is a bloody hard thing to go through, especially knowing you are completely innocent of anything. Make sure you use family and friends for support because without them you may just go insane!
Mine ended well for me, they gave me money and a fantastic reference, along with my favourite... the apology
