The problem in the UK is not the population NUMBER but the population AGE DISTRIBUTION.
100 years ago only 1% of the population was over 65, now it is 25% and rising (have not checked these figures so they may be wrong but you get the idea)
Also the birthrate amongst "white" English is dropping.
Now because the working population has to support the older population it means that you have a decreasing working population suporting an ever increasing older population.
The maths do not add up and this is why there is such a major pension problem.
This is why immigration has been encouraged, to bring in young familes who generally breed quicker than the indiginous English population.
The problem with this is that you then have a rapidly growing immigrant population with all the problems that entails.
Have a look at the UK populatoin pyramid here, compared with say Uganda.
http://www.census.gov/cgi-bin/ipc/idbpyrs.pl?c ty=UK&out=s&ymax=250
See how the UK bulges in the middle for people around 40 to 50 years, and decreases below 40 years. These people will all be retiring in 15 to 20 years.
And see how Uganda is wide at the bottom and very narrow at the top, shiwing they have many young people but few old people.