I was wondering about this too, though I didn't donate at that level.
You would either need high quality recording equipment (not that hard to get - free recording software, $200+ mic, $40 usb audio-interface) or to fly to the UK.
Since the concert mentions that you need to provide your own travel, I'm sort of assuming they're expecting the coaching to be via Skype or something and then you just record at home and send in the audio.
As for what character, expect to be someone like Chimpy or Nabnut or Snacker. Minor characters that offer up a single Pagie and some comedic relief. The main characters will almost certainly be voiced by the devs so they don't need to track down old voice actors again for future games.
Make the forum great again.