Harassment by the employer for submitting maternity leave

I told my employer I was pregnant (I have been with the company for nearly 3 years).

My boss told me not to return after the baby is born and also said that I should leave before my maternity leave. I told them it's my right to stay/choose.

Since that day my boss has harassed and bullied me at work. I raised a formal complaint against him.

However, the company are supporting him saying it never happened as they don't want to be liable.

I am now signed off work sick because of the abuse/treatment at work that ended up with me nearly having a miscarriage.

If I resign, I lose my 3 years EOSB, my medical and visa. But if I stay, I am subjected to daily harassment.

What legal rights to do I have?