It is mostly indoors, but there is an outdoors area for the Privet Drive and assorted other bits sets where you can also buy butter beer. You won't spend long there compared to the rest though and I don't remember it being cold inside. Butterbeer was all we bought because food tended to be overpriced as you'd expect. You might also end up paying for photo prints of the green screen broomstick thing because kids (and adults!) find it very cool, so bare that expense in mind. I agree that you should bring a lot of space for pictures, there are so many 'curiosities' that we took loads as well!
It is expensive in the gift shop, there is a cafe, butter beer is a must try( that's from inside not the age in the foyer! Make sure you have lots of room for pics, I took 600 without trying lol! Layers is best, it's not cold inside but there are outdoor parts too! Have fun