Let's begin by acknowledging that there's little objective difference between moving for work from Aberdeen to London or arriving in London for that purpose from Sofia. Other than the matter of language and some unspecified "cultural" distinctions.
Next we need to appreciate that population churn is important to the economy:
It is important to note how this ‘[population] churn’ helps cities. Knowledge-based economies run on the quality of ideas. Ideas are not only a function of intelligence or education, but also the depth of information a person, or a city, receives. Historically, a lack of information – via a lack of demographic inflow – has ‘Balkanized’ social networks in Rust Belt cities. This has led to a culture of parochialism, which has hurt economic development.”
This quote comes from a report looking at Cleveland, Ohio - long seen as a struggling city, even by the standards of the American mid-west. Importantly the work redefines 'knowledge' - rather than see it merely in terms of high order education or skill, it sees it as:
...the set of personal relationships and knowledge of other places and social networks that we all carry to some extent. Global cities not only score well on traditional knowledge measures, but because they are destinations for migrants, they excel in this more broader notion as well
In simple terms, the benefits of migration lie in diversity - if most of your migrants come from one place (as is the case with Bradford, for example), the 'knowledge' advantage is limited. What matters is that the inward migration - as in London - is from a multitude of places.
Thus Cleveland's problem - one repeated again and again elsewhere, not least in England's northern cities - is:
Attraction is very weak. Hence population decline, but also an inbred, closed society. About 75% of the people in metro Cleveland were born in Ohio, versus 30-60% in other, more globalized cities. Among large metros in the US, Cleveland ranks 6th in its percentage of the population living in the state they were born.
Mobility - migration - matters. The central advantage that London has over (most of) the rest of the UK is that it fits this model. People take their ideas, originality, initiative and spirit to the city - it is this rather than any sort of innate, local factor that drives London's success as a great city.
This work should make us stop and think about how we respond to migration. And to consider that by making it a problem we might damage our economic future?