If you want to bring down the national debt, the worst thing to would be to bring net migration down to zero, according to the OBR's 2012 fiscal sustainability report.
Figures from the Office for Budget Responsibility illustrate that zero net migration would itself (independent of the policies needed to achieve it) have negative fiscal consequences in the short and long term. Under a zero net migration scenario it estimates that within the next five decades public debt would double to nearly 200 per cent of GDP compared to the expected level of debt under current policy.
By contrast, in a high migration scenario the OBR predicts that the long-term health of the public finances would dramatically improve, without the need for reforms to either taxation or public spending. A similar outcome could also be achieved by higher fertility rates.