Yes, you can, there are loads of free channels broadcast on the Sky platform - but be aware that not all the terrestrial channels (1,2,3,4,5) are broadcast "in the clear" on Sky. 4 and 5 are currently "free to air" but not "free to view" via the Sky platform - they are still "encrypted" and need decoding to be viewed on the Sky system which requires a viewing card in the box!. You don't need to actually subscribe to Sky, just have a viewing card present in the box.
This means you would need to use either a) your old Sky card or b) a "FreeSatFromSky" card for �20. { THIS IS NOT THE SAME AS FREESAT - TO CONFUSE YOU EVEN MORE) or c) someone elses old viewing card that they no longer use. If you are buying a second hand Sky box from Ebay or similar then ask if it has a viewing card with it,
"Free to Air TV Channels
You will need a viewing card to view certain channels on Sky Digital. Currently these include Channel 4, Five, Five Life, Five US and Sky Three - but the list may vary in the future.
BBC1, BBC2, BBC3, BBC4 and ITV1, ITV2, ITV3 and ITV4 are all free to air channels and can be watched without a viewing card (with a Sky Digibox and a correctly aligned satellite dish of the recommended size). "
You could always buy a FreeSat box instead of a Sky box. Uses the same signals and your Sky satellite dish, look here;
http://www.freesat.co.uk/index.php?page=whatso n.Main
This is a useful page which should answer any other questions you might have about this;
http://www.stevelarkins.freeuk.com/freeview_on _Sky.htm