The ice covered almost the whole of the land of the British Isles and so the region was completely barren. The only species that may have survived being Arbutus Unedo (Strawberry Tree) a native of southern Ireland.
As the ice retreated the land was colonised initially by species such as Birch and Scots Pine with the last arrivals being species such as Beech and Hornbeam. We can visualise the trees and other plants migrating northwards as the climate improved. Once the English Channel was formed this migration ceased leaving the 33 native trees.
This migration continued within the British Isles with the result that Scots Pine which probably was found all over the region can be said to now have a habitat in parts of Scotland only.
Page 4.0 - Definition of British Trees