flash is the key...
If you're in the uk the BBC will direct you to the flash radio service which is unusable if you're using dial-up; to access the real audio stream use a browser that doesn't have flash and you'll get the old lower bitrate real audio file which is intended for listeners outside the uk.
For some reason the BBC Iplayer help pages choose not to relay this information:
"In the UK can I play radio programmes with RealPlayer or Windows Media Player rather than Flash?
No, because BBC iPlayer detects whether you are in the UK and selects the appropriate player as described here. We aim to give you Flash as this gives the best sound quality."