The Leavers say they want to prevent EU citizens coming to UK because some of them might be migrants, and other people coming from the EU who are not EU citizens because some of them might be migrants.
If you want to prevent people coming to the UK from the EU you can only do it if you have border controls.
The UK has a border with the EU where the Republic of Ireland meets Northern Ireland. Possibly at some future date there might be an EU border where the Kingdom of Scotland meets rUK.
If the Leavers want to control immigration, they must have border controls, and they must control all points of entry, which includes the NI border.
If they don't control borders, they are not controlling immigration.
What does Nigel's favourite word "Control" mean, if not that? His core voters want to keep foreigners out.
Has somebody said it is impossible? Who?
The Outists say they will not introduce border controls between the Republic of Ireland and the UK. If this is true, how can they control entry from the EU into the UK?