Weights of things will vary as to what equals one cup, but as far as using a pyrex container, you can absolutely use the UK ones. One US cup equals 8 fluid ounces in a pyrex measuring cup. Some measures are different here, but fluid ounces are the same. So if you even wanted to use that for things like flour you can fill it to the 16 oz line on a cup and that will be the same as having filled it to the 2 cup line on a US liquid measure container. Obviously, this is a more difficult way to measure than just using the cups though as you can't precisely go across the top to get one cup.