Did you apply formally under flexible working? Hopefully you have covered what you need/why as well as how your new hours will work at the company - offering possible solutions to any issues that may occur by you going part time (like temporary cover or a job share or distributing workload).
Can you clarify their grounds for turning you down as well please. There are 8 reasons that they can use and they need to explain the business reasons behind it.
You are right that they have 14 days to hear the appeal then 14 days to let you know the outcome. Potentially you will have to return to work full time.
You are entitled to return to the same role (unless not reasonably practicable - say the role no longer exists) and same terms and conditions as before.
I suggest you ask for clarification regarding your role, your desk, your secretary etc so you know exactly what is going on when you return.
Constructive dismissal - there has to be a serious breach of contract such as a breakdown in the implied trust and confidence that is so bad that you can no longer work for them - the relationship has to be untenable. Can't say that this would be the case for turning down flexible working - again depends on the reason they turned it down.
Have you got details/dates/names etc of the crap treatment that occured whilst you were pregnant as that would be useful.
You would have to follow the formal company grievance procedure to try and get a resolution to the issues and only once that is completely exhausted could you claim constructive dismissal. (but note that something so serious, even if its one isolated incident, could be enough to undermine the contract and cause you to resign instantly - though you would still be expected to raise a grievance formally after leaving!)
Might they be saying you are redundant, is it something you would be willing to consider at this moment in time - possibly a compromise agreement with a pay off?
Sorry about your childs issues, thats enough to worry about no doubt as it is.