RE: The Muslim population of London is 12.4% as noted.
London has approximately the same population as New York City, i.e., 8.2 Million.
New York City has just 70,000 Muslims, which amounts to 0.875% of the population.
If London is 12.4% Muslim, that makes it over a million Muslims.
If what you say is true about London, the total number of Muslims in London will make it the one of the top 10 cities in America by population.
See here:
http://en.wikipedia.org/wiki/List_of_American_cities_by_population
That’s more people than San Francisco, San Jose, Boston, Seattle or denver.
And London is not the UK, anymore than NY or LA is the US.
Cities by their nature and history have always been cosmopolitan, so you will always get much more of a social/racial/religious mix than you will in towns or villages or rural areas.