Watching TV on anything except your TV can actually be quite a difficult job. There are services such as TVCatchUp but it's very slow and the quality is not so good.
FilmOn is a free iPad app which lets you watch TV. The quality is definitely very watch-able and the sound quality is great. Obviously it takes up quite a lot of bandwidth so I only recommend watching stuff over WiFi. It's a great app and I've been using it a lot since finding it.