Not everyone gets Sky1 in the UK?
No, Sky1 is included with basic subscription satellite (Sky Digital) and cable (Virgin Media) services. They have the rights to other things like Lost, 24, Battlestar Galactica, Prison Break etc as well. Think of them as equivalent to a channel on basic cable in the US.
Sky1 (or Sky One as they used to be known) have had the first run rights to every Stargate series. SG-1 and Atlantis are a big part of their schedule, they show at least one episode almost every day, and those repeats typically perform well for them.
Sky do sell on the rights to show episodes to broadcast channels. In the past Channel 4 have shown Stargate SG-1 and Five showed Atlantis but neither performed particularly well. Generally, people who want to watch the show aren't going to wait for terrestrial repeats anyway.