Business is booming at Olsztyn-Mazury airport in northeast Poland: Its two weekly flights, both to the U.K., are booked up all summer and almost full for winter. Then it’s a trip into the unknown. With the clock now ticking on two years of Brexit negotiations, Poland looks more vulnerable to a painful divorce between the U.K. and European Union than anywhere else. Poland is the biggest net recipient of EU aid and also the continent’s largest provider of cross-border labor. And it’s in the arrivals halls at provincial airports like this former military base 160 kilometers (99 miles) north of Warsaw where those two things meet.