that is also the problem, that for want of a better word the indigenous population are not having more children, as you pointed out, many simply can't afford it. Though the more recent arrivals from the sub continent, Africa are, some of those who may not be contributing as much, get more out of the public purse by child benefit, tax credits, housing benefit and whatever else is on offer. Time that things like child benefit, was capped at two children.
I also wonder that your idea of millions more people are needed, where do they live, and what jobs are on offer for millions more, more low paid ones, so we fill up our cities ever more, we are already straining at the bit now, think what our services will be like having to cope with those extra millions.