Genome: An archive of BBC listings from 1923 to 2009
Pointed out by curiouswombat, Genome is an archive of BBC listings from the Radio Times magazine (BBC TV and radio listings) from 1923 to 2009. You can find out what was supposed to be on the air in Britain any day or time within this period - it won't necessarily reflect changes were made after the listings were published, of course. This includes program descriptions, cast lists, regional variations etc. etc. It doesn't include commercial stations etc.
later: As is pointed out in comments, there are numerous OCR errors which are being handled by a communal wikipedia-style editing process - don't rely on this 100%! However, they explain this on the front page, so hopefully nobody will make any horrible mistakes.