Approaching the end of the Cretaceous period, where on the planet (latitude and longitude) was Cornwall, England? .. Approximately.
The rocks that make up Britain have been slowly drifting north over geologic time. About 700 million years ago it was near the South Pole!
I paused this Youtube video at 75 mya, held the end of a toothpick over Cornwall, then played it to the end keeping the toothpick steady over that point, under which the approximate location of Bilbao, Spain eventually slid itself. Bilbao is at 43°15′25″N according to Wikipedia.
So now, think of how longitude is defined: Relative to the meridian through Greenwich. A cursory glance at the video I linked to shows the rocks that make up Great Britain have moved as a unit and haven't rotated appreciably over the last 75 million years. So the longitudes associated with Cornwall haven't changed much in that time.