Unless you have signed a legally binding settlement agreement with your employer waiving your right to bring - for example - a constructive dismissal claim against them (it sounds like you haven't done this), then you can pursue any legal option available to you.
As PP have said, Constructive Dismissal can be difficult to prove. Unlike Unfair Dismissal, the burden of proof is on you. Usually the advice is to go through the grievance process before you resign.
You mention two things: Discrimination and stress. With a discrimination-based constructive dismissal claim, it is important that you resign immediately after the discriminatory act. If you left a period of time between the act and your resignation, even just a few weeks, you could be seen to have accepted the breach.
With stress, you must establish that it was caused as a direct result of your employer's breach. You should be able to evidence your attempts to resolve the situation with them (either by grievance or otherwise), and that you felt forced to resign primarily as a result of the stress their breach caused. Again, your resignation should have been prompt.
It's also incredibly important that your resignation made clear that you considered yourself constructively dismissed. There's a difference between leaving because you feel that you can't/don't want to work somewhere anymore, and establishing that a serious breach of contract has forced you to leave.
Nobody can say for certain without all the facts, but this is where your lack of grievance and email stating that you had no intention to take further action may come back to bite you.
You should take legal advice immediately - they'll be able to tell you if you still have a case.