In-episode example: When everyone at CDMS are forced to wear school uniforms, everyone, except Dickie (who is British of course), Adam and Principal Pixiefrog, started speaking in obviously fake British accents.
Speaking of Tom Kenny, David Coppertrout also qualifies.
Then, there's Jake's cousin Lord Saucypants, voiced by Rick Gomez.
Keep Circulating the Tapes: Two DVD volumes of the show do exist but they're Region 4 and region locked. The entire series IS available on iTunes however, and the first season is available on Netflix.