MARGARET WARNER:

Then-Prime Minister David Cameron called last year's Brexit referendum, hoping it would fail, and boost his Conservative Party's prospects in the next general election.

But after the surprise result, Cameron stepped down, leaving its execution to May. The U.K. has two years to hammer out the specific of its withdrawal from the 27-nation bloc. But face-to-face talks aren't expected to get under way until late May. And the negotiations promise to be tough.

British Finance Minister Philip Hammond said he expects some give and take, but added: