If I resign, can I claim constructive dismissal?
If you resign because your employer's behaviour towards you amounts to a fundamental (i.e. very serious) breach of contract, you could claim constructive dismissal. In the majority of cases, you will need two years of continuous service to bring your claim. There are, however, exceptions. For example, no service is needed to bring a claim where you resigned because of unlawful discrimination.
The treatment you have received must be so serious that a tribunal is likely to agree with you that it goes to the root of your employment contract. An example could be a failure by your employer to pay your wages.
Very few tribunal claims for constructive dismissal succeed. Lots of things can go wrong. Take very careful advice. Even if you win, unfair dismissal compensation awards are nowhere near as high as many people think. The median award for unfair dismissal in 2016-17 was just £7,512.
Very strict time limits apply. Also note that Acas early conciliation applies to all tribunal claims.