I am sorry your little girl had an accident and got cold on the way home.
Take heart from the fact that it is not rare for 6yo in Y1 to have accidents on occasions at school.
Sadly schools do not have a ready supply of spare clothes, especially in larger sizes. They rely on donations from parents. Often these spare clothes are not returned either. This is why schools remind parents to return them asap and why they often resort to PE kits.
The getting cold thing - well, tbh, this is a bit of a mix of both imo. School obviously thought she'd be fine. You didn't take waterproofs either despite knowing it was hailing. Whilst it would be ideal if you'd been told I am sure your DD will be ok. If it wasn't so wet to warrant waterproofs over tights then I am sure cold legs, whilst not pleasant, will be forgotten quickly.
We do often ask children to try not to go to the toilet in lesson time, especially during the carpet time, when the beginning/end of lessons occur. But we also do encourage not leaving the class unless an emergency at other times too. I think many parents don't really realise how disrupting it can be when a series of children decide they need to visit the toilet, one after the other, when a lesson is taking place. It disturbs the other children's learning, as well as the child involved. It is also a well known distraction technique too.
We do make sure children know they can go in emergencies - this is not reiterated every single lesson I admit, but it is the general rule. It would appear that your DD was not aware that she would be able to go in an emergency. That is bad planning/instructions by the person giving the message out, I agree. Was this an unusual situation and not something that normally happens?
I am sorry to hear that your DD is suffering at school and being bullied. I suspect this fact is probably making this incident seem far worse to you than it was for your DD at the time. That is understandable.
Have you spoken to your DD to reiterate that she MUST always ask if she is desperate, even if the general rule is that they should not go? If not, I would do this now. And then tbh I'd leave it now. You say your DD is not bothered by it any longer. I'd follow her lead in this. If she was still upset then maybe I'd follow it further.