Lweji
You're forgetting population growth
Every country needs children or migrants. What you don't want is an increasingly elderly population that can't be supported by those in work
You are using the ageing population argument as a jutification for mass immigration. Thi is an argument which has been debunked time and time again.
Furthermore you are mendaciously quoting Migration Watch statistics when MW certainly does not support large scale immigration, quite the contrary, they want to greatly reduce immigration and use a points system.
Let me explain why immigration is not a solution.
The demographic age balance of the past has been based upon a population which is both continuously growing, and where people died on average at a significantly younger age than now.
Now I am sure you will agree that it is simply unsustainable for human populations to grow indefinitely, so at some point, we have to face up to the reality that populations are going to be substantially 'older' than now. Even in Third World countries with their youthful populations this 'ageing' of the demographic is beginning. China is anticipating difficulties and it wont be long before other Asian countries are too.
Consider too that if Britain sucks in young immigrants from countries such as Poland, which themselves have rapidly ageing populations, then who is going to support the 'old Poles'?
And secondly as immigrants age too, and as our new bloated population begins in say three to four decades, to face exactly the same problems that we face now, we will be even worse placed with a larger and poorer population to cope with this economic shift. There is a real risk it would lead to calls for culling (murder) of the elderly when they can no longer take care of themselves (as ever the wealthy elites would be exempt).