I would go to sky and ask to cancel because you can't afford it, they will offer you about a 20% discount normally, the same if you are with virgin media, I do it all the time, don't think I have ever paid the full price for their package.
And I like Sky TV by the way, people slag it off etc, but it's got loads of documentary programmes that I enjoy, and lots of stuff for the kids, plus their sports package on there is second to none, it's my one thing I indulge in as I don't smoke, don't drink (pub on home games excluded) and don't collect stamps or anything lol
BT Vision is crap by the way, my sister had it, and it's expensive for what you get, you also need to be near the telephone exchange for it to work properly, virgin media is ok but again it's expensive if you add everything together.