"Yea but past migrations happened, therefore you cant complain about the scope and scale of any change, which we implicitly agree will happen in this argument if you think about it"
 
Sort of a 'cant achieve any solution because it isnt absolutely perfect to an arbitrary level of abstraction and historicity'
 
That because you cant claim there are indigenous Britons, or we're all mutts, or whatever then you cant refuse Pakistanis and Somalis. Of course the group which made the nation-state we call Britain or the UK does exist and has been contiguous