The UK uses "units of alcohol" -- basically the %ABV multiplied by the volume, which gives you the alcohol content:
A 25ml shot of a 40% abv spirit is 25 x 40 / 1000 = 1 unit.
To get the units of alcohol in a drink, sum the units of alcohol for each ingredient //
1.8 - 2.0 is at the light end for a cocktail ; 2.6 - 3.0 getting hefty ; 3.6 - 3.9 caution required, and the upper end for anything on Difford's Guide (that I have seen).