What you need to prove a constructive dismissal case
Proving contructive dismissal can be difficult and many claims don't win. It's important you get help and advice. The citizen's advice bureau has openly said that if you take your claim to an employment tribunal without help and advice, you're likely to lose.
There are several actions you must take to have a good chance of winning your claim.
Speak to management
You will need to show proof that you tried to resolve things before you were forced to resign.
As a starting point, you can speak to your direct manager. If you need to go to upper management you should. You can also talk to a trade union representative or someone in the HR department.
For each of these interactions take records and keep all of your correspondence. If you can correspond in writing, by email or letter, that's best.
Follow your company's official complaints or grievance process
Find out what the formal process is and follow it as closely as you can.
Log your complaint officially and in writing, keeping all of your notes on any discussions or meetings.
Record all incidents
Every time there is an incident that you believe to be part of your claim for constructive dismissal, make a record of it. That might be a log or a journal with dates, times, the people involved, any witnessses, and all the details.
Keep supporting documents
Carefully keep any relevant documents that back up your claim such as payslips if its pay related, time sheets for a case to do with work hours, company memos, emails or any other documents that could be revelant to the claim.
Gather witness statements
Make notes of any other colleagues who happen to be around when incidents occur. Once you have left the company you can then ask them for statements. It will help if you can provide them with dates, times, locations of where the incidents occurred.
You must resign
You can only claim constructive dismissal once you have been forced to resign. If your employer terminates your employment, it may be unfair dissmissal but it cannot be constructive dismissal.
There must have been a breach of employment contract by your employer
There are many possible reasons for this including workplace bullying, discrimination, and many other causes.
Your resignation must take place in a reasonable time frame
This is open to some interpretation but the longer you leave it, the greater then chances a tribunal might decide you waited too long.