I notice that the audio in Freeview TV programmes is AAC. Do Freeview re-encode the original AC3 audio (which is what's broadcast on freesat) to AAC or do they re-encode the master audio (uncompressed?) to AAC and then broadcast that?

AAC is 128 Kbps
AC3 is 384 Kbps or 192 Kbps depending on the channel.

Which gives better quality?