Here’s the answer to my own question about how "native plant" is defined for England:
Plants in Britain can be classified as native, naturalized, alien or cultivated. There are around 35 species of native
tree, for example, depending on how certain sub-species are counted.
These plants were already here or arrived here naturally – that is,
without Man’s help – after the last Ice Age, about 11,000 years ago.
Britain was still part of mainland Europe and plant species slowly
moved north-west into it as the climate improved and the ice retreated
northwards. (There were only about 200 species here at end of the Ice
With the gradual
warming of the world’s surface, the ice began to melt, raising the
seas. Eventually, about 8,000 years ago, the English Channel flooded.
This stopped the natural migration of plants from the rest of Europe,
one consequence of which is that we have only about half as many native
species as France, for instance. (Scientists know this from pollen
deposits and fossil remains.) This date is taken as the cut-off point
for deciding whether a plant species in native or not. Examples of
native trees on the reserve are Ash, English Oak and Field Maple.