It’s not Eel Jelly. It’s actually Jellied Eel. The difference is that with a name like “Eel Jelly” it would seem to be a dish made by adding Eel to Jelly. But Jellied Eel is actually eel cooked in such a way that it becomes jellied. Eels are naturally high in collagen, so you don’t need anything special to make them jellied.

Back in the day that was a bonus because eels were poor people food, and the fact they became jellied resulted in a bit of natural preservation, which was important for poor people’s food in the days before refrigeration.

As for the spices used, I imagine before colonization it was whatever naturally grows in the UK for the peasants, and whatever grows naturally in Europe for the nobility, who were often tied to or from European noble houses. Here’s an article on some of the things that can be foraged in the UK. I imagine that a lot of the spices that used to be used are no longer used. They were probably replaced with better spices that don’t have a bitter aftertaste, or have a stronger pleasant taste.