Why does everyone assume that we need to replace our aging population and combat falling birth rates?? By all means let the population fall back to 3 million! The falling birth rates is a natural reaction to extreme discomfort and horrid standard of living in this miserable little island, and it must be allowed to restore itself to a more fitting population number! Already sinkees are leaving in hordes - so either way nature will balance itself in some mysterious way!!
Don't focus on total population, focus on output/earnings per person so the young people can still support the old!!