Can I clarify why you believe it might affect you please?
Your question talks of somebody else stealing, not you, and I'm not sure why you might feel you can not longer remain in the company?
I am a senior manager in the organisation and the person in question is one of my managers. The item that they stole is a voucher with a cash value of £40. The job that they do involves cash handling , invoicing and preparation of financial claims in the region of £3million per annum. if they were to return to their post I could not trust them and the integrity of their work. in my opinion returning to work would make my position as their manager untenable. The theft is not in question as they admitted it verbally directly to myself. I then reported it my manager who is Chief Executive who instigated the investigatory process
Okay. Ultimately the decision on what to do with the individual concerned is that for the company, i.e. your employer.
You do not have to agree, or like the decision, but it is out of your hands in legal theory. You could of course speak with your employer, make your feelings known, and that you do not trust the person concerned and that dismissal ought to result. But the decision is for the employer.
I am quite clear that the decision is for the employer.
My concern is that if the decision is not dismissal then I would not be able to continue working in the organisation and would have no option but to resign
I don't think it's likely that you would be able to argue constructive unfair dismissal in these circumstances though.
At the end of the day, it's the employer's business that she is in, and that you serve. Hence, ultimately, if the employer wishes to retain her, it cannot be a breach of your contract that would entitle you to walk away and claim constructive unfair dismissal. It would be a simple resignation on your part.
If you tell the employer this, then they need to make a decision who they would rather keep.