Brexit will not affect the Eurail Pass
You can still travel with a Eurail Pass in the UK - this won't change as a result of the Brexit. There are also no restrictions for travel in 2020 between the UK and the European mainland.
There could be changes in travel regulations to and from the UK from 2021 onwards. Check the UK Government advice to learn more.
You have already created a cart and therefore the currency cannot be changed anymore!