To base an argument for or against immigration on numbers alone is missing an important factor – population age. The UK population is ageing rapidly; a third of us will be over 55 by 2025. With a declining birthrate from the late 80s to the early 2000s and an increase in women waiting until their 30s and 40s to have children, this poses problems. Who will look after us in our old age? Who will run our essential services? Who will fill the skills gap that we are already experiencing?

Here's a section from a BBC article:
"In a dramatic and unprecedented demographic shift the number of young people is dwindling while the older sector of the population rapidly expands.

The underlying cause is that we are living longer and having fewer children - well below the replacement rate of 2.2 per woman - but the size of the baby boomer generation, who are just starting to retire, is accelerating the trend.

By 2014, projections suggest, over-65-year-olds will overtake the under-16s.

And by 2025, the number of over-60s will have passed the under-25s for the first time. "

For me, the answer is obvious: we need immigrants. Anyone have a better answer?