Also moving species within a country is a lot less problematic than moving them between countries, as legally-speaking they might not have been native within the 'new' boundaries (England/Svotland) compared to the old (UK). Could add extra red tape for White-tailed Eagle (obviously formerly resident in recent history in Scotland, much less clear-cut in England).
Bearing in mind the problem of climate change, and the way that habitat fragmentation will hinder the northward shift of species, expansion of English species into Scotland could now have a political dimension, or at least a practical one if we need to translocate e.g. butterflies or birds into new areas further north - you're now dealing with two nation states and cross-border international 'introductions', not translocations within borders. Then there's the issue of funding such conservation measures - which nation pays? England for giving away its species, or Scotland for taking a novel 'invasive' species?