3G really shouldn’t be faster, but it’s possible. It’s also possible that it’s dropping to lower bitrate because it’s on a mobile connection.
Are you trying the same station in both cases?
Have you tried using the iPad when it’s right next to your wireless router? Streaming audio and video is particularly sensitive to a borderline wireless connection.
It’s also possible that there’s a network problem between BT and the source of radio station. Just because you get 19Mbps to a speed test site, doesn’t mean that you can get 19Mbps to everywhere else on the internet.
“Buffering” is just the process of loading up the next few seconds of audio so that the music can carry on playing through minor network interruptions. If the data is arriving at your ipad slower than it’s being played then the music has to stop whilst it refills the buffer.