You should absolutely be able to go to the toilet with one of the examiners as a chaperone, particularly if you are pregnant or have a medical condition. It’s unhealthy to hold off going to the toilet if you need to. Just raise your hand and say if you need to go
They actually can’t do that. You are entitled to use the bathroom. Make sure you speak up for yourself.
Good luck with your exams! I imagine you have said you are pregnant as you say you weren’t able to get an exception. I would t of thought they could do that. Surely they could facilitate with a chaperone or something. Otherwise I would only take very small sips of water to try and avoid a too full bladder. The peeing usually gets better as baby moves more up toward the abdomen. The body is weirdly good at adapting and getting through uncomfortable situations. In between exams and after I’d drink lots of water and pee lots so you keep everything flushed out to avoid any risk of uti etc.
They can't refuse you to use the toilet while pregnant! I had this when I sat my uni exam. Are they aware your pregnant?