The accepted answer to this question states that you shouldn't expect a roast to generate much in the way of juices left behind in the pan, and that accords with my experience (which admittedly has always been very small roasts).
But if that's the case, why do (British) gravy recipes very commonly expect you to have significant quantities of meat juices to start to make gravy with? For example, here ("up to 200 ml meat juices") and here (unspecified amount of juices and "2 tbsp meat fat").
It feels like I'm missing something obvious that these recipe writers expect readers to understand. Any thoughts?