SINGAPORE (Reuters) -Oil prices slipped on Monday on weak China data, but held on to most of last week's gains, as investors awaited U.S.-China trade talks in London later in the day, hoping a deal could boost the global economic outlook and fuel demand. Brent crude futures slipped 18 cents, or 0.27%, to $66.29 a barrel by 0644 GMT. China's exports growth slowed to a three-month low in May as U.S. tariffs slammed shipments, data showed, while factory-gate deflation deepened to its worst in two years, heaping pressure on the world's second-largest economy both at home and abroad.