I happen to believe the government have not managed this at all. It is a collective responsibility not just for the UK but for the world.
It is a finite place.
Some countries manage population very openly e.g China. Others manage it via other ways.Uncontrolled population growth by whatever means can lead to many problems.

For example how much energy do we need to produce to meet the electricity demands. This is not just academic.Where will all the extra oil & food come from? What social changes do we need to plan for in terms of maintaing the value systems of the host country as well as assimilation of the new immigrants.
We know too well how internal tensions in some countries can surface.We should aim to avoid these problems.

Whilst an absolute number would be crass so would no target. The aim of a balanced migration policy over a periods seems sensible, this would need an indicative number upon which to sensibly plan and aim for.

The fact is the government needs to be able to control its borders effectively, by using restoring entry and exit controls.
It could by reviewing & amending the EU agreement on free movement, it may be neccessary for a manged cap process for EU residents.We could agree that with our EU partners,who already do this, we would be hardly xenophobic to suggest this.

The government also needs to enforce the current law, it needs to plan & manage much better, its poor record in this respect helps to feed irrational fears.
eg it needs to remove far more failed aslyum seekers & assist with aid to the effected countries direct.

I fear that no effective planning & control is unsustainable and that spin for electioneering is unsustainable.

I disagree that managing this via a target ‘Cap’ is not in the UK society interests, its appears balanced and sensible. The government can take whatever power it needs, possibly subject to amending the law and some treaties/constitutions. It may need to improve it and its agency’s performance somewhat though.