I recommend Rackhams book, Woodlands. Very good!
Here is some information from page 28 on natives. (roughly)
Native plants are species which reached this country by natural meals while Britain joined to Europer about 7000 years ago. eg Ash and Badger
Naturalised species propagate themselves without human intervention - eg sycamore and fallow deer
Exotics - most - do not spread where people have put the. Eg sweet chestnut will spread and walnut will not.
Naturalised species can go into 2 groups. Archaeophytes - historic but not native like sweet chestnut and neophytes- modern introductions like Norway Maple.
Sycamore counts as a neophyte.
Whether it is native or not is in a muddle - depending on whether or not conservationists approve of it!
Plants need not be native or alien everywhere - eg Scots pine
That's all for now - it is a VERY long book!