Sorry to hear this icecreamlady.
That link Betsy's posted is a really good one and easy to understand. As Ruth says, it makes clear that it's always better not to resign. If you were to bring a constructive dismissal claim a tribunal would look at whether you made every effort to resolve the situation first, including bringing a grievance before resigning if at all possible.
Having a chat with the CAB might be a good idea, and I would advise giving ACAS a ring as well, Betsy's given you the number.
It would be constructive dismissal you would have to claim, and it's difficult to prove and can be stressful. As Ruth says, think carefully about what you want to achieve. Now that you've resigned, what do you want to do now? What do you want to get out of any action you take?
I'm going to be honest and say that bringing a constructive dismissal claim might not be the best option. Talk about it to ACAS and the CAB as obviously you can give them more information than we have here. But proving it is very difficult, it will be very stressful and will take over your life for a while, and you need to be very clear what you want out of it.
If it's some money you want, you may get it. But compensation would be more or less based on how much money you have lost. You are also expected to 'mitigate your loss', in other words do whatever you can to get another job. If you do get another job, your actual financial loss will be very small so pursuing your employer for compensation is unlikely to be worth it.
Talk to the CAB and ACAS about what your options are and think them through, talk it through with your DP if you have one as well and work out the best route for you to take.