The way to look at it Milk has circa 5 Gram Carb per 100 mL, small biscuit maybe 10 Grams, as a Type 1 diabetic I would need circa 1.5 Units of Insulin for that. So depending on how high your personal Insulin resistance is it will impact your reading to a different degree
Well whether it's 'permitted' or not depends on what information you are hoping to get out of it really. If you are trying to determine how the meal affected your levels 2 hours later, then having something else within that two hours will skew the results. I drink a milky coffee and have a small biscuit when I am a bit low in order to raise my levels fairly quickly (within half an hour or so), so yes, it is likely to affect your readings at the 2 hour mark.
Well the short answer is yes it will affect your reading- but more importantly as @KK123 asks- what information are you hoping to get out of your reading? A lot of my readings are just to keep an eye on my general levels so can be quite random. If I am testing a particular food/meal I am more careful about the time. I think you want to work out why you are testing- having a milky coffee after a meal you know won't raise your levels tells you about its affect. The problem is that so many things can affect your levels.
I am still a fan of testing but work out why and that will have an influence on when and how often you test.