Not sure if it'll just work if you cancel, though you could always get a digital TV decoder quite cheaply. However since you've got the Sky+ box, look at this:
http://www.freesatfromsky.co.uk
You can get plenty of free 'no monthly fee' content. I *think* Sky might want to charge you about £20 for a one-off change to a Freesat viewing card. Still, not bad value if you can keep using your dish and the Sky+ box.
Disclaimer -> I work for Sky, but not at the TV bit. No idea if this is a good service or not!