2) I'll bet something like Google's favicon cost WAY more than that.
3) 600 pounds isn't all that much when consulting work hourly rates are compared against it, and is practically nothing if they paid an actual designer.
It's easy to say "I could do that in 5 minutes." (cite: from the email exchange) But, could you? Including the design? And making sure after you shrunk it down it still looked okay? There's a good chance you'll need to go in there and draw each pixel by hand to make it look right (and it better be right).
Favicons are important, it's a mistake to believe they are always a 5 minute project.
(edited to add "always" to the last sentence)