I think it's really complex, and a mix of factors.
Firstly, first generation immigrants tend to be the people with 'get up and go' by definition, so they're likely to have high aspirations for their children.
Secondly, some cultures - most notably China and India - put a very high value on education.
But there is, I'm sure, a 'London effect' from the huge amount of funding and effort that has gone into the Capital's schools. If you look at GCSE results by ethnicity over time, there has been a very definite change. This report shows GCSE results by ethnic group from 2006/7, and then 2010/11.
In 2007, Pakistani, Bangladeshi, Black Carribean and Black African origin children all got worse GCSE results than White British origin children. By 2011, all groups had improved, but the non-White British had improved by considerably more. Evidently that pattern has continued, and now they've pulled ahead. IMO it's very likely in part if not entirely because London schools have improved massively, and non-White pupils are concentrated in the capital.