I've seen many Flixbus routes between London and mainland Europe.
I wonder how Flixbus gets across the English channel; is it using the train or using some ferry?
According to a Twitter response from 2016, Eurotunnel at least was the usual way for FlixBus.
The review here indicates both ferry and Eurotunnel train are possible for FlixBus.
Whether a particular bus takes the ferry or Eurotunnel can be checked (at least sometimes) by looking at the schedule, but the mode of crossing could still change depending on delays etc.
For example, this bus is (supposed to be) taking the tunnel, taking 40 min from Folkestone to Calais
This bus instead is taking a ferry, taking 1 hour and half to cross from Dover to Calais
As of 2020, it varies between services; in order to read about a specific bus, search here (selecting your destination first). Information will appear for both directions.
FOCA and CAFO means the Eurotunnel is used; otherwise the ferry is used.