There are so many variables that it's impossible to provide meaningful estimates. The general advice is that, while you would have to do a great deal of basic surfing (the first half of your list) to break the 10GB limit, downloading or streaming media (especially video) will soon bust it.
For example, 4 laptops and 4 smartphones in our house would generally use around 15MB. The recent addition of a couple of iPads for moderate video streaming has doubled that. Any gaming or downloading of films would put our 40GB limit in real danger.
If you're only just exceeding 10GB why not upgrade to 40, save the worry and get more freedom? Otherwise you can check your monthly usage to date at http://bt.com/broadbandusagemonitor and slow down as necessary.
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