UK is not in the Schengen area. Everyone needs a passport to move between UK and the Schengen area, even today.
You may or may not need a visa from March 2019. Nobody knows this yet. There could be agreement that a UK or EU passport is enough to cross borders, there could be agreement that a visa is required (and some UK newspaper already phantasized about visa costing £50), there could be agreement of something like ESTA, there could be no agreement at all and you get arrested on sight if you arrive in the wrong country. Nobody knows yet.
Same treatment as for Swiss citizens seems a reasonable goal that everyone should be able to agree on.