There is some difficulty here. The words SEEPED and STEEPED have at least some association and similarity. In the Free Dictionary Online definitions, one sees
SEEPED as "diffused gradually"
and
STEEPED as "saturated" (as in, getting thoroughly wet--BB)
SEEPED suggests that over history, a few events have been experienced
in the British Isles.
STEEPED suggests that every part of the British Isles has been thoroughly affected
by many changes.
For me personally, I hear STEEPED more often. We speak of making tea, as a process
of STEEPING the tea leaves in hot water.
It is possible that the author was using SEEPED appropriately, but I have both read STEEPED and heard it as the more common and appropriate verb.