I suggest you read the original article,i, it covers your questions.
http://www.bristol.ac.uk/cmpo/publications/papers/2014/wp333.pdf
I'm not generalizing anything here, but reiterating factual statements from the BBC article. London's school children do well, why, because of immigration. That doesn't explain anything about other cities or immigrants, no one said it did.
I have read the original article and if it's conclusions were true the results would be transferable elsewhere. But that doesn't happen. Which goes back to my edit there and about the specific nature of the immigrants and whether it is really fair to compare London to anywhere else. Let alone make the sweeping generalisations that are being made in the media and in this thread.
" It is argued that ethnic minority pupils have greater ambition, aspiration
, and work harder in school. This is the main argument here – London has more of these pupils and so has a higher average GCSE score than the rest of the country"
If that was true then why are Bradford and Leicester not up there?