You can watch in on FX channel (it's on quite late) if you have Sky and they also occasionally show it on Channel Four. It is also available on DVD, if you don't know that already
Up until fairly recently, it was on BBC2 on Saturday nights, along with Seth McFarlane's other show, American Dad. However, I'm not sure if it's on anymore.
Sorry potterfan, It used to be on really late before about 1am. I checked the time after I posted my previous note and realised it was on at nine (and a double bill too!) and I thought you'd think I was mad - nine o'clock is halfway through my day. I love the show especially Stewie and Brian.