Yes - per liter of pure ethanol. If you buy a pint of regular lager beer you are buying basically 0.56L times 5.2% alcohol which is 2.9 cl pure ethanol, or as a brit would some times say "about 3 units". So a minimum proce per unit is the same as a minimum price per litre of pure ethanol. The minimum price per liter of ethanol is 100x the minimum price per unit (since one unit is 1 cl pure ethanol).

The reason this is so fantastically confusing is because sometimes even regulators use "alcohol" to refer to the drink, and not the ethanol.

Basically. There is a minimum price set of 0.50 per unit. This means a bottle of whisky, 70cl at 37.5% ABV cannot retail for less than £14 whereas previously these would be on promotion sometimes at £11/£12/£13.

By setting it per unit instead of quantity of drink the government targets super strength cheap cider etc that were linked to lots of anti-social behaviour and crime.

How about an offer of 15 with a 3 pound non-transferable (therefore worthless) store credit?

Per unit. The effects of a litre of vodka isn't the effects of a litre of cider.