https://www.bbc.com/news/business-49263781
Britain is going to leave the Eurail/InterRail scheme on 31st of December 2019. It has nothing to do with Brexit, it's a decision made by the British rail companies independently.
Holders of InterRail or Eurail passes will still be able to travel to and from London on Eurostar - but not beyond St. Pancras station anymore.