There's Immigrants and there's immigrants

The people who use that statistic, add up all the money brought in (& then the money earned  off those assets) by Immigrants , thats all those rich (& a lot of Mega-rich taking advantage of Osbornes lax tax laws on them), Arabs, Russians, Europeans, Australians, South Africans, Americans, Brazilians etc who move to London and bring in many many £billions and also drive up the prices of the London housing market .

But most people count immigrants as the less well off, who come in a take the low wage, unskilled Jobs and these numbers have always dwarfed the first kind,

So  statistically, the Country is better off, but you already know what they say about statistics

Immigration figures are very hard to read, because they include Brits returning to the UK ( like the many 1000s who return every year from Spain in recent years) and ALL the foreign students that enter the country