What speed do you get when checked with independent speed testers?
Ensure that you're laptop is connected to your router with an ethernet cable (not wirelessly) and that nothing else [such as a smart TV or games console] is using your internet connection. Then run the test here:
http://www.speedtest.net/
Repeat here:
http://www.bbc.co.uk/iplayer/diagnostics
Then try both again with your laptop connected wirelessly. (If there's a significant difference you'll need to change its channel).
If your figures are very different to Sky's you should challenge them about it. Also, assuming that your connection is via conventional copper cables in your phone line (which is what a speed of 3.3Mbps suggests anyway), ask Sky which
type of broadband they're supplying to you. The basic service that BT Wholesale sells to other ISPs is 'ADSL Max', so that's probably what you've got. But a decent ISP should, possibly after a bit of hassling from you, be able to get that upgraded to 'ADSL 2+', which roughly doubles the available speed.