Yes you can use the box but .....
You need a feed from the dish and so the charge is probably partly for running the cable plus fitting a new part to the dish (if needed) to support your extra box.
If you have an old Sky Viewing card you will be able to watch all the Free to View: BBC, ITV, Ch4, Ch5 etc. If not only the Free to Air stuff: BBC, ITV, If you want true multi-room (watch all your subscription stuff) you need to pay Sky £10 pcm.
t, June 2008