if your using a sat box for tv over svideo, you should not be scanning for channels.
I am not sure if it is the same for sky as it is for Dish, but you should have a channel grabber that gets you listings for you if it is. Then you will have to configure myth with the TuneToChannel.sh so that LMCE can change the channels. search the forums and wiki for this, it is explained pretty well in there.
Scanning for channels is for tuners and ota capture so that is probably where you have your wires crossed.
Get started here:http://wiki.linuxmce.com/index.php/Setting_up_MythTV
Hope this helps,