Right now I can already go to the EU and fill my car with booze until the axles threaten to break. And then bring it back into the UK without paying any duty, so long as it's for private consumption.
But there's an extra electioneering fiddle. HMRC will forego the alcohol tax on booze sold on the Channel crossing so it will be cheaper than it is now.
So currently you buy as much alcohol as you like anywhere in the EU and the duty is included so you can bring it back duty free.