Not true, of the 6.68m population of London, in total 1.45m were born outside the UK and of that 1.45m, Pakistanis and Bangladeshis represent the third and fourth largest ethnic groups after the Indian and Black Caribbean groups. So in total non-UK born Bangladeshis and Pakistanis make up less than 10% of the vote.
I do however agree that Ken is a scumbag, but a vote for him is not seen as anti-american, it is seen as anti-Blair. He was voted in because Blair did everything in his power to stop him winning and Londoners don't like being pushed around. They are however, now sick of him and will undoubtedly boot him out at the next election!