If you believe current net migration levels are sustainable, is there a point at which you'd believe it would become unsustainable?
Yes, I do believe that it would eventually become unsustainable, but I think we are a very long way off that point at present. I also think it would be self-limiting, as people would eventually choose not to migrate (or to go elsewhere).
Personally, I wouldn't support FOM at a worldwide level right now, as the numbers would be unmanageable and the economic inequalities are simply too great at present - much, much greater than within the EU. However, I would support a relaxation of our current visa requirements for countries outside the EU. Particularly for groups such as students, who contribute a huge amount to our economy - I'd like to see them taken out of the immigration stats altogether tbh.
There is still economic inequality within the EU, of course, but it is not as stark as the difference between the uk and some other parts of the world. Personally, I think that we probably opened up to FOM a little too quickly with regard to Eastern Europe, but I understand that there were strategic reasons for doing so and I don't accept that it has been the disaster that many people like to say it has.
The ideal of the EU is to bring up the living standard in all its member countries to a comparable level. We are not there yet, but the difference is not so extreme as to make the current levels of migration unmanageable. If greater equality could be achieved at a worldwide level, I would love nothing more than to see FOM worldwide.