Which is not the same as 20 US fluid ounces. (It’s about 19.3 US fluid ounces). An imperial fluid ounce of water weighs about 1 ounce. A US fluid ounce of water weighs about 1.03 ounces. (Yes, the ounce as a measure of weight/mass is the same in the two systems).
But not in ONE gulp matey.
I’ve done the yard of ale business and it took a helluva lot more than one gulp.
It also took me longer than 16 seconds and I was sick afterwards 
I’ve seen one or two UK measuring cups using volume as a measurement in the form of cupfuls. Which I can only assume is approximate unless there is a standard cup that her majesty’s government uses 
Cooking isn’t a chemistry experiment though. The measures in most recipes aren’t exact and good cooks approximate and adjust as they go. If the recipe author got good results with one cup of flour, you probably will too no matter what it actually ways.
I don’t understand how weight is easier either. When you measure by volume, you just pour a certain amount into a cup. Going by weight, don’t you have to pour and weigh?
Yes, but with some things you get a different amount by volume from day to day. Take flour: sifted all purpose flour will take up much more volume than non-sifted (or semolina flour, for that matter). As a result, most large baking recipes will rely on weight as that is a more accurate measurement than volume.