Well obviously if you're going back to the retreat of the last glaciers, then nothing in Britain is native - not flora nor fauna. By 'native species' most of us mean the varieties you would see on any woodland or hedgerow walk, which should grow well under local conditions. There's no point keeping double-double pink cherry trees if you want to feed wildlife on pollen, nectar and fruit. Some recent research has shown that flying insects prefer native species to garden species of flowers - no clue as to why but it clearly matters.
I'm with you though on having some modern cultivars of trees and shrubs in our hedgerows, especially hazelnuts - even when ours appear to be full and plump coming up to harvest, there's never anything harvestable inside. I wouldn't mind squirrels getting them, as to me wildlife is wildlife and we shouldn't have favourites, if the nuts would only grow properly. So maybe I'll get a good cultivar for a crop. I love the idea of a monkey puzzle tree - there are a few in our area so we know they can be grown here. What are the seeds like to eat?