Probably the handiest way to watch these is with the BBC iPlayer if you live in the UK or the RTÃ‰ player if you live in Ireland. As far as I'm aware of, both will let you watch programmes up to a month of them being broadcast and you can watch them as they stream, rather than waiting for them to download first.
If you really want to try downloading an episode of something by Torrent and are aware of the legal issues, try a Google on the programme title followed by the date (e.g. 26th February 2010), followed by the word "Torrent". If you don't get any useful results, try writing the date in another format, e.g. 26-02-2010, 26-02-10, etc. If you still have no luck, then that particular episode or programme is unlikely available as a Torrent, at least on a public indexing website.