What you're saying Fitzy is that the UK could possibly hang on to what it already has, but it's going to have to throw something else on the table even to achieve that.
Overall immigration is highly unlikely to decrease, until there's no 'pull' factor, ie the economy has gone completely down the pan.
Apparently 50,000 EU citizens left the UK in the last quarter(?). There are reports that these were the more skilled - doctors, nurses, teachers, etc and that unskilled immigration increased slightly. I can't blame skilled people from leaving. They have highly marketable skills and obviously don't want to stay in a country, where their future is uncertain and they feel unwelcome.
There are already restrictions on benefits in place, which stop freeloaders turning up and expecting a hand out. The UK could have gone further with restricting benefits, but the government chose not to.
The UK has never lost control of its borders, because it was never part of the Schengen Agreement. However, any kind of further restrictions are going to involve billions of pounds in enforcement - all to "take back control". I agree, ww. What a lark!