The core issue
The question is whether the employer committed a serious breach of contract, whether you resigned in response to that breach and whether you acted without affirming the contract.
Evidence that matters
Grievances, emails, messages, meeting notes, medical evidence, HR responses and resignation wording can all matter. Timing is especially important.
How to avoid weak framing
A resignation caused by stress or poor treatment is not automatically constructive dismissal. The ET1 needs to identify the breach and connect it to the resignation.