JONATHON PORRITT, one of Gordon Brown’s leading green advisers, is to warn that Britain must drastically reduce its population if it is to build a sustainable society. (Link)
The current population is usually quoted as about 60 million - ie, almost double the number we can actually feed. The "carrying capacity" of these islands is only 30 million people. We actually have a population density twice that of China!
However the real population may be higher still since this number doesn't correctly include the vast immigration since the last census in 2001. Food consumption statistics from the grocery supermarkets tend to indicate a population closer to 70 million already.
But if it ever falls below 30 million we should definitely see about getting some immigrants to settle here.