lucky to have a BT vision box already. For anyone else, cancelling Sky is an option only after the first year (or possibly after the last upgrade/ change... eg if you added Sky HD) otherwise there may be some penalty charges if they have a 12 month contract with you.
AwesomePan Sky was doing a 'FreeSat from Sky' deal where they'd install for 150 quid, and the main thing that someone wanting to switch from the premium channels/monthly sub to freesat option, would be to get Sky to change your viewing card. Best to ask them about this. For someone without Sky there may be a fee of 20 quid. If you have Sky they should be able to just downgrade your card to block the Sky 1 etc channels, but leave radio and BBC, ITV etc.
If you've had Sky for a while, is change to a FreeSat HD box, so for the channels they offer in HD you would only pay out once for the more advanced box, and not have a tenner a month extra for HD the way Sky provides it...
You can also consider changing the LNB (lump at the end of the arm on the dish) to plug more cables in (some have 2 sockets, but you can get 1, 2, 4, 8 sockets) so adding a FreeSat box in another room, or in a bedroom, is easy - get 8 cables and can add extra boxes in other rooms later. Admittedly not the Sky channels but you also add BBC Radio like 1 Xtra, 6 Music, 4 Extra, and decent quality for 5 Live (and 5 Live Sports)
Note that you need 2 cables for a box which can record, before you think I'm mad to suggest using 8 cables! (Oh, but make sure the dish has a strong arm on it :)
Ponders Thanks for info on Foxsat. As for Virgin doing satellite... it would be possibe but for any company to rent space on the satellites is costly. Sky can afford to because they have the Movies and Sport to bring in money (admittedly a lot goes out to buy the rights to show things) but it has taken 20 years to go from making a loss each year to bring in a profit. Virgin took over 2 or more loss making cable services (Telewest ? and another). Not sure if they took those debts on as well, but they probably had a struggle to make the cable go from loss to profit, and doubt they could find enough customers not already on Sky to switch to satellite if they had tried that. Also might have fallen foul of some competition rules because they would be seen as trying to muscle in on both land and sky services.