EU referendum, brexit and the aftermath

I suppose this is of little relevance now but a friend (a history professor) pointed something out to me yesterday.

I knew that the UK doesn't currently exercise the controls available to it to limit EU immigration (means testing, health insurance, etc etc) but what I didnt realise was that it could be implemented regionally.

I.e under current EU legislation, the UK could have made it so the areas of the UK under the perception of immigration pressure could have legally had immigration levels slashed, all without leaving the EU.

Puzzling.