What does "cresting with" mean? Does the phrase "already cresting with Brexit and T" mean "already being very busy in dealing with Brexit and T"?
Simply a guess here. The definition of "crest" seems to be "reach the highest level". I am not sure.
We did the business quickly: getting Bannon’s support for a Theresa May visit to Washington a week after inauguration. To spin out the meeting, I asked him about rumours of his links to some of Europe’s far-right parties: the Front National, Alternative für Deutschland, Lega Nord. Bannon feigned surprise at the question: “of course” he was talking to them. They were part of the great populist wave that was sweeping over western democracies, capitalising on the complacency and corruption of ruling elites, and already cresting with Brexit and Trump’s election. Next stop would be a victory for the Front National in the French elections in May 2017.