There's some really incorrect advice on this thread.
You have to resign to bring a constructive dismissal case, so whoever said you can't bring it because you have resigned is wrong.
You need two years service for constructive dismissal but not for pregnancy discrimination, so it needs to be a discrimination claim, not a constructive dismissal claim. Which, if the OP had actually seen a lawyer as she claims, she would know.
CD just for being called to face a disciplinary would be highly unlikely to succeed.
The disciplinary is probably weak for them, and for them to dismiss on that would be harsh, but they know the OP can't bring an unfair dismissal claim so they don't care about that.
The rest does sound like discrimination based on the pregnancy and the previous pregnancy. BUT, and again, I'm pretty sure the lawyer will have explained this to the OP, the claim is very likely out of time. The claim needs to be within three months (three months minus one day) of the events being complained about. The most recent event being complained about seems to be last August.
So, in summary, no claim can be brought.