A fixation on reducing numbers leaves no room for rational discussion of what that means for the economy and society British political debate has long been dominated by public anxiety about rising levels of immigration. How might that change if the population tide were to turn? Not at all, would appear to be the answer. Net migration has in fact been falling since before Labour came to power last July, and yet there has been no end of demand for ever tighter controls and no end of government acquiescence. New figures published this week by the Office for National…