Freesat from the BBC/ITV will work with a Sky dish, so you could cancel Sky then re-use the dish.
If you want to carry on using Sky (not sure why you would want to unless it is for HD) I think you have to fit an extra device (a Quad LNB) to the dish.
Yeah I am not a techy, but when I saw this post earlier I tried to find the answer to your question. Ended up getting very confused as there is a Sky Freesat as well as the BBC/ITV one, but one thing I did see was that the BBC/ITV one has a smaller footprint (just for the UK), so presumably is on another satellite, and also presumably is somewhere else in the sky.
As you have Sky, why get another, as some - if not all - are bound to be duplicated?
Ring Sky and speak to them, I rang them about HD and they only wanted �99 for the box plus �60 installation which is �150 less than quoted on the website.