Personally, I would think that you might have a claim for constructive dismissal (leaving, based on having employment changes forced on you).
However, I would strongly recommend you gain as many facts as possible (with regards the changes to your employment contract), and discuss your situation with your local Citizens Advice Bureau.
If I were in your situation, I would think that the writing is on the wall, and start looking for alternative employment � do you really want to continue to work for an outfit that treats you like that?