If you do buy a sky+ box, you will also need a twin LNB which will have to be installed on your dish. Not beyond the competence of the average diyer provided your dish isn't on the roof. You will also require cable as a second cable is required for the second tuner in the digibox. Finally if you are not subscribed with sky and still want to use sky+, they will charge you £10 per month for the privilage of recording.
Go for freesat, you can get twin tuner recorders for them, sky is overrated, for the most part you are paying for channels you do not even watch because of the way their packages are bundled.
Even if they wont let you cancel, they cannot stop you downgrading to the bare minimum package until your contract is up. The broadband is free even with their smallest package, given they have no fair usage policy, so it's truly unlimited downloads, it's still not bad value.