Seriously dude, why 70 million?

Before Christmas, I asked 'Why 70 million anyway?' in response to the tabloids' constant hammering of the number as an unacceptable level for the UK population.  I keep seeing the figure, but no reason why it should be so scary.

"I risk looking like a total fool if it turns out that everyone knows if the population hits 70 million, the demons start emerging from the mirrors or something, but sod it, I'm confused.  Why have we settled on that number as the one that must not be crossed, ever?"

I said.