'In London in 2011, 45% (3.7 million) out of 8.2 million usual residents were White British'
This statement from the article tells you nothing about immigration, necessarily. It tells you about 'usual residents' of London, many of whom will be foreign students, temporary workers, and medium-term residents etc, who will return to their country of origin or move on to another country at some point.
London is the world's capital city, as New York was in the 80s and early 90s and somewhere else (probably Shanghai) will be in due course. People are being rather unreasonable to take all the benefits to the UK's economy/international standing etc of that while issuing coded and not so coded complaints about all the foreigners cluttering up the place.
They aren't all a burden by any means. Some of the foreign people of colour are educated and rich. Many of them keep the NHS going.
And bear in mind that a lot of migration is from EU citizens with an absolute right to freedom of movement within the EU. Not all EU migrants are white, by a long chalk. I know quite a few French migrants of North African ethnicity who've come here because it offers them far more opportunities than at home.
Of course, we have the same right, and I have never noticed a thread on Mumsnet suggesting all the British expats moving to Spain, Cyprus, Greece, Bulgaria or wherever to build new lives (generally without learning the language or about the host culture) were a blight on the countries they moved to.
By all means discuss immigration and whether there needs to be a limit and where but remember to take the rough with the smooth.