I don't think a Dragon Cam can receive the stay alive signal for a S*y card, so you will need a digibox to update the card every few weeks, I think the Dreambox has this ability, but I'd check it out first.
S*y usually have promotions, approaching Christmas, free installation, half price subscriptions etc., but you will have to have the digibox connected to the telephone line for 12 months.
The good news is that I think you're allowed 2 dish installations, under planning regulations, so you can have a mini dish for S*y and still keep your motorised for more choice.