A "unit" of alcohol is 1cL. This amount is commonly used in the UK -- the amount of alcohol in a drink is often printed on the label in "units", and people have some ideas around how many "units" one can drink safely. It's used in personal health education at school.

The minimum price was set to 50p per unit / centilitre of alcohol.

(Keeping easy metric amounts for simplicity. In bars beer and cider is sold in 568mL glasses, though wine and spirits are nice round metric amounts.)

A 1L bottle of vodka, at 40% ABV, contains 40cL of alcohol. The shop or bar must sell it for at least 40×£0.50 = £20. In England, Tesco's cheapest 1L bottle of vodka is £15, so this has increased the price by £5 in Scotland. A fancy vodka which costs £22 in England is unaffected.

A 2L bottle of cheap cider at 5% ABV must cost at least £5. At Tesco in England, this is available for about £2.