Interesting question.
Mercedes was imported from Churchill(Canada)- date? 1984?
Before her I can only think of the ones that were received at London circa 1960's- I believe at least one of their pairs were a gift from Russia.
Some of the earlier ones at the other zoos that kept them (Chester, Dudley, Flamingo Park, Chessington, Bristol, (London), Whipsnade, Edinburgh etc) were probably imported but then produced cubs at least occassionally, so there was no need for anywhere much to further import Polar Bears.