I just did a quick unscientific comparison of the raw output from the Sky box and the Radio X online stream.
The Sky box puts out MP2 (MPEG1 Layer II) files, at 160kbps, 48kHz.
The official Radio X web stream is MP3 (MPEG-2 Audio Layer III) at 128kbps, 44.1kHz
Because we don't know how many and what chains the audio goes through before it gets to the Sky box, any number of factors could affect it compare to the "official" stream.
To me, the Radio X web stream sounds better. I think the Sky audio is a little overdriven, causing mild clipping/distortion.
The web stream sounds cleaner and smoother.
Even though the Sky stream is at a higher bitrate, it uses MP2, which is less efficient, so the MP3 should be of higher quality:
A rule of thumb is that Layer II audio requires at least 192kbps or so while Layer III reduces bitrate demands to 128kbps, to achieve comparable audio quality.
This is all moot if the Radio X web stream falls over and we only have one source, but this may be of use for future reference.