Milk is sensitive to temperature fluctuations so it needs to be kept in your fridge at all times. Try to avoid storing it in your fridge door as that is the warmest part of the fridge.
You can consume milk after its best before date, providing it still looks and smells fine. Milk begins to visibly curdle when it goes off, so if it is bad, you’ll know about it. In New Zealand, it is unsafe to consume sour milk.
If you have bought too much milk, you can freeze it for when you need it. Make sure you tip a little into your cuppa before you freeze the bottle so the milk has room to expand.