Yes, nobody wants to be doing imperial>metric conversions when in mid-recipe, nor the other way round. Many British books give both sets of numbers, but the conversions are generally not too accurate. It can become important to use only one set of measures in a given recipe.
I suspect that metric weights are taking over, as that is what children have been taught for some years, so that few of those under say 30 years are comfortable with imperial measures. I have an electronic scale in the kitchen with a switch to toggle between the two. Add in the use of metric/imperial/'gas mark x' for oven temperatures and there is little wonder that text-book cookery can seem like a minefield.
Out of curiosity I just compared my two different sets of measuring cups, one thinks a cup is 250ml, the other thinks 189ml, so even cups are evidently a variable. Having checked online, I find that a metric cup is 250ml, a US cup approx 237ml, and a Canadian approx 227ml. Heaven knows where the 189ml measure comes in.
It is lucky that quantities are truly critical in only a fairly small number of dishes.