"...the economy needs large scale immigration to support the ever growing ageing population,"
Sorry mushroom, but as has been touched upon, this strategy is ludicrous. The main reason for this (apart from the reasons you have mentioned) is that, incredible as it may seem, those being shipped in to support the pensions scheme eventually get old and need pensions themselves. And guess what? They also need pensions. So in comes another lot who er.. eventually get old. I'm sure you get the idea.
The country needs to develop an economic model which does not rely on an ever increasing population (which, in fact, will have to be an exponential increase and is clearly not sustainable. It also needs to set to work those who will not do so. It is utter madness to have to rely on ever increasing numbers, many of whom have to be brought in from abroad, to sustain the country's economy.