There is a collection of GBA cheats in the cheat section of GBA temp.
I could never get cheats to work on my iPlayer though. I'd be interested to hear if you happen to get them too.
EDIT: Looks like you ran into the same problem as me. I'm beginning to think that the cheat function was never implemented in the iPlayer's GBA emu. Everyone has advice on how to do it, but I haven't found anyone who's actually got it working.