The information requested falls within the responsibility of the UK Statistics Authority. I have asked the authority to reply.
Letter from Karen Dunnell, dated November 2008:
As National Statistician I have been asked to reply to your question, regarding what, on the basis of the most recent population projections, is the level of immigration at which the population of the UK would remain below 70 million on unchanged assumptions about birth rates and mortality. (236159)
The information requested cannot be calculated directly from the latest national population projections published by the Office for National Statistics in October 2007. These projections included a principal (or central) projection, and a range of variant projections. The variant projections included a low migration variant and a zero migration (natural change only) variant.
In the principal projection, which assumes a long-term net inward flow of +190,000 persons a year, the population of the UK is projected to reach 70 million in 2028. In the low migration variant, which assumes a long-term net inward flow of +130,000 persons a year, the population of the UK is projected to reach 70 million in 2035. In the zero migration variant, which shows the effect of the principal assumptions of fertility and mortality in the absence of migration (or where migration inflows and outflows are equal at every age), the population of the UK is projected to reach almost 64 million in the year 2032 and then start to decline.
Assumptions for national population projections are conventionally expressed in terms of net migration (immigration less emigration) rather than for the gross flows separately. However, nominal immigration and emigration totals are used in the projection process mainly to enable plausible age distributions to be calculated for the assumed net migration totals.
It is apparent from the 2006-based projections that a level of annual net inward migration, somewhere between zero and +130,000, could be assumed at which the population would remain below 70 million. However, there are a range of combinations of immigration and emigration levels which could achieve this outcome. For example, a lower level of immigration could be used than in the 2006-based projections but with no change to emigration.
Alternatively, both immigration and emigration could be reduced. In practice, it is likely that lower levels of immigration than those experienced in recent years would, in time, lead to a reduction in the level of emigration as well.