Trump on Brexit

A meeting with British Foreign Secretary Boris Johnson was to be on Syria and other issues. Kerry also held a low-key meeting with the Archbishop of Canterbury, Justin Welby, in which the two discussed international reconciliation.

For Britain's current leaders, however, the focus on Monday was more on the remarks that Trump has made in favor of Brexit, the British voters' decision to leave the European Union, and the new administration's assurances that new and better trade deals are on the horizon.

"We hear that we are the first in line to do a great free trade deal with the United States," said Johnson last week after getting those assurances during a trip to the U.S. ahead of the inauguration.

The remark reflected new optimism following the Trump victory after outgoing President Barack Obama last year warned Britain it would go to the back of the line on trade deals if it voted to leave the European Union.

WATCH: Johnson Looks Ahead to Trump Administration

Merkel waiting

In the interview, Trump blasted the decision by German Chancellor Angela Merkel to call for Britain and other European Union members to accept migrant quotas and an open door policy in her own country.

"I think she made one very catastrophic mistake and that was taking in all of these illegals, taking all of the illegals from wherever they come from and nobody really knows where they came from. You'll find out," Trump said.

Asked about Trump's criticism of her migrant policy, Merkel told reporters, "I am personally waiting for the inauguration of the U.S. president. Then of course, we will work with him on all levels."

In remarks Monday, Kerry told CNN journalist Christiane Amanpour he thought it was "inappropriate" for a president-elect of the United States "to be stepping into the politics in a quite direct manner." Kerry said Merkel has been "extremely courageous" in her response to the refugee crisis, which he acknowledged has "had some problems."