I always considered them to be weeds. They'd grow on the Moon given half the chance and have taken extremely well to cold acid soils of the North Highlands of Scotland. It's a fine tree, a tall tree, a handsome middle eastern type tree - the 'Omar Sharif' of trees. The Sycamore was introduced into this country over six hundred years ago.
Theories vary, as usual.
It is possible that the Romans brought it over. My preferred theory is that it was deliberately introduced by a group of people who travelled all the way from Scotland, England and Wales through Europe to Palestine and back. And the reason they brought back the seeds of the Sycamore, rather than the Olive or the Palm was that the Sycamore was considered sacred. It was this beautiful, slow-growing tree which acted as the boundary marker between Upper and Lower Egypt. It was beneath the broad spreading branches of this tree that Mary and Jesus stopped for a breather on the flight into Egypt.